res_pjsip: Add disable_tcp_switch option.

When a packet exceeds the MTU, pjproject will switch from UDP to TCP.  In
some circumstances (on some networks), this can cause some issues with
messages not getting sent to the correct destination - and can also cause
connections to get dropped due to quirks in pjproject deciding to
terminate TCP connections with no messages.

While fixing the routing/messaging issues is important, having a
configuration option in Asterisk that tells pjproject to not switch over
to TCP would be useful.  That way, if some glitch is discovered on some
other network/site, we can at least disable the behavior until a fix is
put into place.

AFS-197 #close

Review: https://reviewboard.asterisk.org/r/4137/
........

Merged revisions 427129 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@427130 f38db490-d61c-443f-a65b-d21fe96a405b