LRU_CYCLE_PERIOD is defined,but not used.

The idea is that very few commands have a lot of keys, and when this
happens the allocation time becomes neglegible.

now that we may use it more often (ACL), these excessive calls to malloc
and free can become an overhead.

The callback approach we took is very efficient, the module can do any
filtering of keys without building any list and cloning strings, it can
also read data from the key's value. but if the user tries to re-open
the key, or any other key, this can cause dict re-hashing (dictFind does
that), and that's very bad to do from inside dictScan.

this commit protects the dict from doing any rehashing during scan, but
also warns the user not to attempt any writes or command calls from
within the callback, for fear of unexpected side effects and crashes.

This makes simpler to give people help when posting such kind of errors
in the mailing list or other help forums, because sometimes the
directive looks well spelled, but the version of Redis they are using is
not able to support it.

Use built-in alsoPropagate mechanism that wraps commands
in MULTI/EXEC before sending them to replica/AOF

in some cases we were trying to kill the fork before it got created

it seems that running two clients at a time is ok too, resuces action
time from 20 minutes to 10. we'll use this for now, and if one day it
won't be enough we'll have to run just the sensitive tests one by one
separately from the others.

this commit also fixes an issue with the defrag test that appears to be
very rare.

seems that github actions are slow, using just one client to reduce
false positives.

also adding verbose, testing only on latest ubuntu, and building on
older one.

when doing that, i can reduce the test threshold back to something saner

I saw that the new defag test for list was failing in CI recently, so i
reduce it's threshold from 12 to 60.

besides that, i add / improve the latency test for that other two defrag
tests (add a sensitive latency and digest / save checks)

and fix bad usage of debug populate (can't overrides existing keys).
this was the original intention, which creates higher fragmentation.