See #7401.

`clusterStartHandshake` will start hand handshake
and eventually send CLUSTER MEET message, which is strictly prohibited
in the REDIS CLUSTER SPEC.
Only system administrator can initiate CLUSTER MEET message.
Futher, according to the SPEC, rather than IP/PORT pairs, only nodeid
can be trusted.

We want to send pings and pongs at specific intervals, since our packets
also contain information about the configuration of the cluster and are
used for gossip. However since our cluster bus is used in a mixed way
for data (such as Pub/Sub or modules cluster messages) and metadata,
sometimes a very busy channel may delay the reception of pong packets.
So after discussing it in #7216, this commit introduces a new field that
is not exposed in the cluster, is only an internal information about
the last time we received any data from a given node: we use this field
in order to avoid detecting failures, claiming data reception of new
data from the node is a proof of liveness.

Reloading of the RDB generated by

    DEBUG POPULATE 5000000
    SAVE

is now 25% faster.

This commit also prepares the ability to have more flexibility when
loading stuff from the RDB, since we no longer use dbAdd() but can
control exactly how things are added in the database.

- the API name was odd, separated to two apis one for LRU and one for LFU
- the LRU idle time was in 1 second resolution, which might be ok for RDB
  and RESTORE, but i think modules may need higher resolution
- adding tests for LFU and for handling maxmemory policy mismatch

* Introduce a connection abstraction layer for all socket operations and
integrate it across the code base.
* Provide an optional TLS connections implementation based on OpenSSL.
* Pull a newer version of hiredis with TLS support.
* Tests, redis-cli updates for TLS support.

cluster.c - stack buffer memory alignment
    The pointer 'buf' is cast to a more strictly aligned pointer type
evict.c - lazyfree_lazy_eviction, lazyfree_lazy_eviction always called
defrag.c - bug in dead code
server.c - casting was missing parenthesis
rax.c - indentation / newline suggested an 'else if' was intended

See for reference PR #6337. Thanks to @git-hulk for spotting this.

Related to #5154.