This commit also adds a top comment about a subtle behavior of mixing
blocking operations of different types in the same key.

Some times it was not released on error, sometimes it was released two
times because the error path expected the "di" var to be NULL if the
iterator was already released. Thanks to @oranagra for pinging me about
potential problems of this kind inside rdb.c.

fix int overflow problem in freeMemoryIfNeeded

An implementation of the
[Ze POP Redis Module](https://github.com/itamarhaber/zpop) as core
Redis commands.

Fixes #1861.

Modules Timer API: fix wrong raxInsert() usage

This is useful in the reply and timeout callback, if the module wants to
do some cleanup of the blocked client handle that may be stored around
in the module-private data structures.

In some modules it may be useful to have an idea about being near to
OOM. Anyway additionally an explicit call to get the fill ratio will be
added in the future.

Plus freeMemoryIfNeeded() refactoring to improve legibility.
Please review this commit for sanity.

This way it is possible to use conditional compilation to be compatible
with a larger amount of Redis versions, however note that this breaks
binary compatibiltiy, so the module must be compiled with the
corresponding redismodule.h file depending on the version of Redis
targeted.

Note that this was an experimental API that can only be enabled with
REIDSMODULE_EXPERIMENTAL_API, so it is subject to change until its
promoted to stable API. Sorry for the breakage, it is trivial to
resolve btw. This change will not be back ported to Redis 4.0.