[NEW] Add caching to COMMAND command to improve performance

[ebarskiy]

The problem/use-case that the feature addresses

COMMAND command is slow as it makes too many string operations
generic clients like to send it on connection introducing unnecessary lag to the system
list of commands rarely changes (via module load/unload operations) making it a good candidate for caching

Description of the feature

Cache command result and invalidate on underling data change (module loads/unloads new command)

Alternatives you've considered

Additional information

two tier caching: per command cache + full COMMAND output

[ebarskiy]

#2839