pjproject: Add cache_pools debugging option.

The pool cache gets in the way of finding use after free errors of memory
pool contents.  Tools like valgrind and MALLOC_DEBUG don't know when a
pool is released because it gets put into the cache instead of being
freed.

* Added the "cache_pools" option to pjproject.conf.  Disabling the option
helps track down pool content mismanagement when using valgrind or
MALLOC_DEBUG.  The cache gets in the way of determining if the pool
contents are used after free and who freed it.

To disable the pool caching simply disable the cache_pools option in
pjproject.conf and restart Asterisk.

Sample pjproject.conf setting:
[startup]
cache_pools=no

* Made current users of the caching pool factory initialization and
destruction calls call common routines to create and destroy cached pools.

ASTERISK-27704

Change-Id: I64d5befbaeed2532f93aa027a51eb52347d2b828