core: Add cache_media_frames debugging option.

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

* Added the "cache_media_frames" option to asterisk.conf.  Disabling the
option helps track down media frame mismanagement when using valgrind or
MALLOC_DEBUG.  The cache gets in the way of determining if the frame is
used after free and who freed it.  NOTE: This option has no effect when
Asterisk is compiled with the LOW_MEMORY compile time option enabled
because the cache code does not exist.

To disable the media frame cache simply disable the cache_media_frames
option in asterisk.conf and restart Asterisk.

Sample asterisk.conf setting:
[options]
cache_media_frames=no

ASTERISK-27413

Change-Id: I0ab2ce0f4547cccf2eb214901835c2d951b78c00