Support enabling redis presence user mapping for much faster presence stats retrieval

[FZambia]

Proposed changes

Relates #750

Currently to get presence stats in Redis engine case, Centrifugo loads entire presence to memory and then calculates num_clients and num_users counters. This may be inefficient for channels with huge number of active subscribers.

This pr Introduces boolean option global_redis_presence_user_mapping. When true:

Centrifugo will maintain additional per-user data structures in Redis when updating presence information (for all channels with presence enabled). Similar to structures used for general client presence (i.e. additional ZSET + HASH). So we get a possibility to efficiently get the number of unique users. This allows improve performance of presence stats API command significantly since we replace calculation of presence stats counters from Centrifugo based on full presence info to Redis based on efficient queries to data structures.

For the channel with 100k unique subscribers number of presence stats ops bumped from 15 to 200000 per second in my benchmark.

This will increase memory usage in Redis, possibly up to 2x from what was spent on presence information before (less if you have extra info in client information attached, since we do not include info payload to user structures).