Modify ast_answer() to not hold the channel lock while calling ast_safe_sleep()

Modify ast_answer() to not hold the channel lock while calling ast_safe_sleep()
or when calling ast_waitfor().  These are inappropriate times to hold the channel
lock.  This is what has caused "could not get the channel lock" messages from
chan_sip and has likely caused a negative impact on performance results of SIP
in Asterisk 1.6.  Thanks to file for pointing out this section of code.

(closes issue #13287)
(closes issue #13115)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@141949 65c4cc65-6c06-0410-ace0-fbb531ad65f3