chan_sip: Session-Expires: Set timer to correctly expire at (~2/3) of the interval when not the refresher

RFC 4028 Section 10
	if the side not performing refreshes does not receive a
	session refresh request before the session expiration, it SHOULD send
	a BYE to terminate the session, slightly before the session
	expiration.  The minimum of 32 seconds and one third of the session
	interval is RECOMMENDED.

Prior to this asterisk would refresh at 1/2 the Session-Expires interval,
or if the remote device was the refresher, asterisk would timeout at interval end.

Now, when not refresher, timeout as per RFC noted above.

(closes issue ASTERISK-21742)

Reported by: alecdavis
Tested by: alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2488/
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Merged revisions 387344 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 387345 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@387369 f38db490-d61c-443f-a65b-d21fe96a405b