Eliminate stale manager events after a set interval, even if AMI clients don't query for them.

Actions (or failures to act) by external clients should not cause memory leaks
in Asterisk, especially when those continued leaks could cause Asterisk to
misbehave later.

(closes issue #17234)
 Reported by: mav3rick
 Patches: 
       20100510__issue17234.diff.txt uploaded by tilghman (license 14)
       20100517__issue17234__trunk.diff.txt uploaded by tilghman (license 14)
 Tested by: mav3rick, davidw

(closes issue #17365)
 Reported by: davidw


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@266682 65c4cc65-6c06-0410-ace0-fbb531ad65f3