res_pjsip/config_transport: Allow reloading transports.

The 'reload' mechanism actually involves closing the underlying
socket and calling the appropriate udp, tcp or tls start functions
again.  Only outbound_registration, pubsub and session needed work
to reset the transport before sending requests to insure that the
pjsip transport didn't get pulled out from under them.

In my testing, no calls were dropped when a transport was changed
for any of the 3 transport types even if ip addresses or ports were
changed. To be on the safe side however, a new transport option was
added (allow_reload) which defaults to 'no'.  Unless it's explicitly
set to 'yes' for a transport, changes to that transport will be ignored
on a reload of res_pjsip.  This should preserve the current behavior.

Change-Id: I5e759850e25958117d4c02f62ceb7244d7ec9edf

configs/samples/pjsip.conf.sample

@@ -810,6 +810,12 @@
                                 ; clients are slow to process the received
                                 ; information. Value is in milliseconds; default
                                 ; is 100 ms.
+;allow_reload=no    ; Although transports can now be reloaded, that may not be
+                    ; desirable because of the slight possibility of dropped
+                    ; calls. To make sure there are no unintentional drops, if
+                    ; this option is set to 'no' (the default) changes to the
+                    ; particular transport will be ignored. If set to 'yes',
+                    ; changes (if any) will be applied.
 
 ;==========================AOR SECTION OPTIONS=========================
 ;[aor]