res_pjsip_outbound_registration: Fix SRV failover on timeout

In order to retry outbound registrations for some situations, we
need access to the tdata from the original request.  For instance,
for 401/407 responses we need it to properly construct the
subsequent request with the authentication.  We also need it if
we're iterating over a DNS SRV response record set so we can skip
entries we've already tried.

We've been getting the tdata from the server response rdata and
transaction but that only works for the failures where there was
actually a response (4XX, 5XX, etc).  For timeouts there's no
response and therefore no rdata or transaction from which to get
the tdata.  When processing a single A/AAAA record for a server,
this wasn't an issue as we just retried that same server after the
retry timer expired.  If we got an SRV record set for the server
though, without the state from the tdata, we just kept trying the
first entry in the set repeatedly instead of skipping to the next
one in the list.

* Added a "last_tdata" member to the client state structure to keep
  track of the sent tdata.

* Updated registration_client_send() to save the tdata it used into
  the client_state.

* Updated sip_outbound_registration_response_cb() to use the tdata
  saved in client_state when we don't get a response from the
  server. We still use the tdata from the transaction when we DO
  get a response from the server so we can properly handle 4XX
  responses where our new request depends on it.

General note on timeouts:

Although res_pjsip_outbound_registration skips to the next record
immediately when a timeout occurs during SRV set traversal, it's
pjproject that determines how long to wait before a timeout is
declared.  As with other SIP message types, pjproject will continue
trying the same server at an interval specified by "timer_t1" until
"timer_b" expires.  Both of those timers are set in the pjsip.conf
"system" section.

ASTERISK-28746

Change-Id: I199b8274392d17661dd3ce3b4d69a3968368fa06