Thanks to @oranagra for the hint about misplaced 'r' flags.

Minor fixes found during merge and code review

Thanks to @tidwall for reporting.
Close #3194.

The test works but is very slow so far, since it involves resharding
1/5 of all the cluster slots from master 0 to the other 4 masters and
back into the original master.

This fixes issue #3043.

Before this fix, after a complete resharding of a master slots
to other nodes, the master remains empty and the slaves migrate away
to other masters with non-zero nodes. However the old master now empty,
is no longer considered a target for migration, because the system has
no way to tell it had slaves in the past.

This fix leaves the algorithm used in the past untouched, but adds a
new rule. When a new or old master which is empty and without slaves,
are assigend with their first slot, if other masters in the cluster have
slaves, they are automatically considered to be targets for replicas
migration.

Fix ZINCRBY return value.

As a side effect, cat commands.txt | redis-cli now is able to handle
lines more than 4096 bytes.

This fix was written by Anthony LaTorre.
The old code mis-calculated the amount of time to wait till next event.

This fix was suggested by Anthony LaTorre, that provided also a good
test case that was used to verify the fix.

The problem with the old implementation is that, the time returned by
a timer event (that is the time after it want to run again) is added
to the event *start time*. So if the event takes, in order to run, more
than the time it says it want to be scheduled again for running, an
infinite loop is triggered.

See issue #3114.

bitops/bitfield: fix length, overflow condition and *sign

We want to report the original command in the stats, for example GEOADD,
even when what is actually executed is the ZADD implementation.

The new bitfield command is an extension to the Redis bit operations,
where not just single bit operations are performed, but the array of
bits composing a string, can be addressed at random, not aligned
offsets, with any width unsigned and signed integers like u8, s5, u10
(up to 64 bit signed integers and 63 bit unsigned integers).

The BITFIELD command supports subcommands that can SET, GET, or INCRBY
those arbitrary bit counters, with multiple overflow semantics.

Trivial and credits:

A similar command was imagined a few times in the past, but for
some reason looked a bit far fetched or not well specified.
Finally the command was proposed again in a clear form by
Yoav Steinberg from Redis Labs, that proposed a set of commands on
arbitrary sized integers stored at bit offsets.

Starting from this proposal I wrote an initial specification of a single
command with sub-commands similar to what Yoav envisioned, using short
names for types definitions, and adding control on the overflow.

This commit is the resulting implementation.

Examples:

    BITFIELD mykey OVERFLOW wrap INCRBY i2 10 -1 GET i2 10

Fixes double conversions in GEO