Eventual replication, including partially async initial confirmations

[vaizki]

NOTE: I am fully aware that the Disque project may be merged into the redis repo
soon as a module of redis 4.2 but I wanted to contribute this idea into disque now
so that this requirement (which is very real for me - your mileage may vary) might
make it into the new & shiny redis module.

First attempt at eventual replication, combining synchronous and asychronous
replication into one ADDJOB operation.

Developed (hastily) to support a scenario with a 2-node system which is
geographically distributed and must support at-least-once semantics but also
operate each site in 'island mode' where only 1 node may be available for
synchronous replication but when inter-node connectivity is restored the jobs
should be replicated to the other node. In this scenario jobs are added with
REPLICATE 2 SYNC 1 to immediately unblock if the local node is alive but
eventually replicate to the other node if it becomes available.

Adds a SYNC parameter to ADDJOB, which is mutually exclusive with ASYNC.

A job with SYNC specified will perform synchronous (block client)
replication to N nodes. REPLICATE will still control the amount of nodes that
are desired for replication.

When ADDJOB with SYNC is received, REPLJOB messages will be sent to the
amount of nodes controlled by REPLICATE, but the operation will unblock only
when N nodes have confirmed that they have replicated. The rest of the
replication is async, replies will be sent to the origin and they will be added to the confirmed_hosts until full replication has been achieved (sync + async).
The confirmed_hosts dict will be released when full confirmation has been
achieved, which stops any future broadcasts.

This additional replication currently happens in the standard awake timer so
the RETRY parameter in effect controls how often the original node attempts to
gain new replications for this job. This may (more likely will) change in the
future.

NOTE: This patch completely respects that Disque jobs are immutable, thus when
the job unblocks it has been written to the AOF and will not gain knowledge of
additional confirmed nodes later. Only in-memory state will change after that.
The on-the-wire replication protocol and AOF format & logic are unchanged, so
a rolling upgrade to this version should be fine.

Because the confirmed_nodes dict is not serialized to the AOF, any node
restarting will have a NULL dict in which case they will not rebroadcast the
job to additional nodes. Thus if the original node accepting the job is
restarted the message may never reach full replication.

Some test cases have been added to cover the basics of the new functionality.

P.S. I made my first ever PR earlier today and hopefully this one will fare better..

[vaizki]

The check that has failed here.. has never failed in my local tests. It's probably because the node is reported as unreachable before the ADDJOB reaches it.. AFAIK has nothing to do with the changes I made but rather with the lagginess of the CI but I'm happy to be proven wrong.

[vaizki]

Also please see my comments in #189 to understand the motivation and use case for this.

[vaizki]

Closing this as we have moved on from disque.. and so has Redis and antirez. Thanks for all the fish!