Update memory metrics for INFO during loading (#7690)

During a long AOF or RDB loading, the memory stats were not updated, and
INFO would return stale data, specifically about fragmentation and RSS.
In the past some of these were sampled directly inside the INFO command,
but were moved to cron as an optimization.

This commit introduces a concept of loadingCron which should take
some of the responsibilities of serverCron.
It attempts to limit it's rate to approximately the server Hz, but may
not be very accurate.

In order to avoid too many system call, we use the cached ustime, and
also make sure to update it in both AOF loading and RDB loading inside
processEventsWhileBlocked (it seems AOF loading was missing it).

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