res_pjsip: Record the serializer earlier on the tdata.

When PJPROJECT needs to do a DNS resolution and there is not a cached
entry available, the SIP request message goes out on the PJSIP monitor
thread instead of the original serializer thread.  Thus when the response
comes back it does not get processed by the original sending serializer.

This patch records the serializer on tdata before passing a request
message to PJPROJECT where it can in Asterisk code.  There are several
places in PJPROJECT for outbound registration and publishing support that
would need to record the serializer.  Unfortunately, without replacing the
PJPROJECT DNS resolver as was done in v14 we cannot fix those without
modifying PJPROJECT.

Even if we backported the DNS resolver from v14, the outbound registration
refresh timer does not go out on a serializer thread but the PJSIP monitor
thread.  Fortunately, Asterisk's outbound publish support doesn't use the
auto refresh timer that would also not go out under the serializer thread.

This patch is v13 only.

ASTERISK-26669
ASTERISK-26738

Change-Id: I9997b9ed6dbcebd2c37d6a67dc6dcee9c78914a4