Move Redis databases background processing to databasesCron().
Actually call databasesCron() inside serverCron().
Only resize/rehash a few databases per cron iteration.

This is the first step to lower the CPU usage when many databases are
configured. The other is to also process a limited number of DBs per
call in the active expire cycle.
Use unsigned integers for DB ids, for defined wrap-to-zero.
activeExpireCycle(): process only a small number of DBs per iteration.

This small number of DBs is set to 16 so actually in the default
configuraiton Redis should behave exactly like in the past.
However the difference is that when the user configures a very large
number of DBs we don't do an O(N) operation, consuming a non trivial
amount of CPU per serverCron() iteration.