RM_Microseconds
Return the wall-clock Unix time, in microseconds
RM_CachedMicroseconds
Returns a cached copy of the Unix time, in microseconds.
It is updated in the server cron job and before executing a command.
It is useful for complex call stacks, such as a command causing a
key space notification, causing a module to execute a RedisModule_Call,
causing another notification, etc.
It makes sense that all these callbacks would use the same clock.
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