Merge branch 'hyperloglog-improvement' of https://github.com/oertl/redis into oertl-hyperloglog-improvement

This is a big win for caching use cases, since on reloading Redis will
still have some idea about what is worth to evict and what not.
However this only solves part of the problem because the information is
only partially propagated to slaves (on write operations). Reads will
not affect slaves LFU and LRU counters, so after a failover the eviction
decisions are kinda random until keys start to collect some aging/freq info.

However since new slaves are initially populated via RDB file transfer,
this means that if we spin up a new slave from a master, and perform an
immediate manual failover (for instance in order to upgrade the master),
the slave will have eviction informations to use for some time.

The LFU/LRU info is persisted only if the maxmemory policy is set to one
of the relevant type, even if no actual "maxmemory"  memory limit is
set.

XINFO is mainly an observability command that will be used more by
humans than computers, and even when used by computers it will be a very
low traffic command. For this reason the format was changed in order to
have field names. They'll consume some bandwidth and CPU cycles, but in
this context this is much better than having to understand what the
numbers in the output array are.

Now you can use:

    addReplyError("-MYERRORCODE some message");

If the error code is omitted, the behavior is like in the past,
the generic -ERR will be used.