MINOR: pools: call malloc_trim() under thread isolation

MINOR: pools: call malloc_trim() under thread isolation

pool_gc() was adjusted to run under thread isolation by commit c0e2ff202
("MEDIUM: memory: make pool_gc() run under thread isolation") so that the
underlying malloc() and free() don't compete between threads during these
potentially aggressive moments (especially when mmap/munmap are involved).

Commit 88366c292 ("MEDIUM: pools: call malloc_trim() from pool_gc()")
later added a call to malloc_trim() but made it outside of the thread
isolation, which is contrary to the principle explained above. Also it
missed it in the locked version, meaning that those without a lockless
implementation cannot benefit from trimming.

This patch fixes that by calling it before thread_release() in both
places.

(cherry picked from commit 26ed183556f3df9e9bde6f6d47143aa4a18ae463)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 46b515ca55b113827ed3e52937cf7ad154d2f6c0)
Signed-off-by: Willy Tarreau <w@1wt.eu>