We usually want to reach the master using the address of the interface
Redis is bound to (via the "bind" config option). That's useful since
the master will get (and publish) the slave address getting the peer
name of the incoming socket connection from the slave.

However, when this is not possible, for example because the slave is
bound to the loopback interface but repliaces from a master accessed via
an external interface, we want to still connect with the master even
from a different interface: in this case it is not really important that
the master will provide any other address, while it is vital to be able
to replicate correctly.

Related to issues #2609 and #2612.

When we fail to setup the write handler it does not make sense to take
the client around, it is missing writes: whatever is a client or a slave
anyway the connection should terminated ASAP.

Moreover what the function does exactly with its return value, and in
which case the write handler is installed on the socket, was not clear,
so the functions comment are improved to make the goals of the function
more obvious.

Also related to #2485.

master was closing the connection if the RDB transfer took long time.
and also sent PINGs to the slave before it got the initial ACK, in which case the slave wouldn't be able to find the EOF marker.

Bug as old as Redis and blocking operations. It's hard to trigger since
only happens on instance role switch, but the results are quite bad
since an inconsistency between master and slave is created.

How to trigger the bug is a good description of the bug itself.

1. Client does "BLPOP mylist 0" in master.
2. Master is turned into slave, that replicates from New-Master.
3. Client does "LPUSH mylist foo" in New-Master.
4. New-Master propagates write to slave.
5. Slave receives the LPUSH, the blocked client get served.

Now Master "mylist" key has "foo", Slave "mylist" key is empty.

Highlights:

* At step "2" above, the client remains attached, basically escaping any
  check performed during command dispatch: read only slave, in that case.
* At step "5" the slave (that was the master), serves the blocked client
  consuming a list element, which is not consumed on the master side.

This scenario is technically likely to happen during failovers, however
since Redis Sentinel already disconnects clients using the CLIENT
command when changing the role of the instance, the bug is avoided in
Sentinel deployments.

Closes #2473.

A few code style changes + consistent format: not nice for humans but
better for parsers.

Instead of manually checking for strchr(n,':') everywhere,
we can use our new centralized IP formatting functions.

Track bandwidth used by clients and replication (but diskless
replication is not tracked since the actual transfer happens in the
child process).

This includes a refactoring that makes tracking new instantaneous
metrics simpler.

RDB EOF detection was relying on the final part of the RDB transfer to
be a magic 40 bytes EOF marker. However as the slave is put online
immediately, and because of sockets timeouts, the replication stream is
actually contiguous with the RDB file.

This means that to detect the EOF correctly we should either:

1) Scan all the stream searching for the mark. Sucks CPU-wise.
2) Start to send the replication stream only after an acknowledge.
3) Implement a proper chunked encoding.

For now solution "2" was picked, so the master does not start to send
ASAP the stream of commands in the case of diskless replication. We wait
for the first REPLCONF ACK command from the slave, that certifies us
that the slave correctly loaded the RDB file and is ready to get more
data.

Same as the original bind fixes (we just missed these the
first time around).

This helps Redis not automatically send
connections from the first IP on an interface if we are bound
to a specific IP address (e.g. with multiple IP aliases on one
interface, you want to send from _your_ IP, not from the first IP
on the interface).

This caused BGSAVE to be triggered a second time without any need when
we switch from socket to disk target via the command

    CONFIG SET repl-diskless-sync no

and there is already a slave waiting for the BGSAVE to start.
Also comments clarified about what is happening.

While the socket is set in blocking mode, we still can get short writes
writing to a socket.

This is useful for normal replication in order to refresh the slave
when we are persisting on disk, but for diskless replication the
child is already receiving data while in WAIT_BGSAVE_END state.

If we turn from diskless to disk-based replication via CONFIG SET, we
need a way to start a BGSAVE if there are slaves alerady waiting for a
BGSAVE to start. Normally with disk-based replication we do it as soon
as the previous child exits, but when there is a configuration change
via CONFIG SET, we may have slaves in WAIT_BGSAVE_START state without
an RDB background process currently active.

Closes #1513

Closes #1537

Info about the replication state with the master added.

The new ROLE command is designed in order to provide a client with
informations about the replication in a fast and easy to use way
compared to the INFO command where the same information is also
available.

This commit adds peer ID caching in the client structure plus an API
change and the use of sdsMakeRoomFor() in order to improve the
reallocation pattern to generate the CLIENT LIST output.

Both the changes account for a very significant speedup.