Allow for reloading SRTP crypto keys within the same SIP dialog

As a continuation of the patch in r356604, which allowed for the
reloading of SRTP keys in re-INVITE transfer scenarios, this patch
addresses the more common case where a new key is requested within 
the context of a current SIP dialog.  This can occur, for example, when
certain phones request a SIP hold.

Previously, once a dialog was associated with an SRTP object, any
subsequent attempt to process crypto keys in any SDP offer - either
the current one or a new offer in a new SIP request - were ignored.  This
patch changes this behavior to only ignore subsequent crypto keys within
the current SDP offer, but allows future SDP offers to change the keys.

(issue ASTERISK-19253)
Reported by: Thomas Arimont
Tested by: Thomas Arimont

Review: https://reviewboard.asteriskorg/r/1885/
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Merged revisions 364203 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 364204 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@364205 65c4cc65-6c06-0410-ace0-fbb531ad65f3