Made MALLOC_DEBUG less CPU intensive by default.

Storing a backtrace for each allocation in anticipation of a memory
management problem is very CPU intensive.

* Added the CLI "memory backtrace {on|off}" command to request that the
backtrace be gathered only on request.  The backtrace is off by default.

(issue ASTERISK-22221)
Reported by: Matt Jordan
........

Merged revisions 397809 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397811 65c4cc65-6c06-0410-ace0-fbb531ad65f3