MINOR: pools: add a new global option "no-memory-trimming"

MINOR: pools: add a new global option "no-memory-trimming"

Some users with very large numbers of connections have been facing
extremely long malloc_trim() calls on reload that managed to trigger
the watchdog! That's a bit counter-productive. It's even possible
that some implementations are not perfectly reliable or that their
trimming time grows quadratically with the memory used. Instead of
constantly trying to work around these issues, let's offer an option
to disable this mechanism, since nobody had been complaining in the
past, and this was only meant to be an improvement.

This should be backported to 2.4 where trimming on reload started to
appear.