Send a PING at the end of the server sync to detect it

Send a PING at the end of the server sync to detect it

At the end of sending all "state" to the remote server, a PING command
is sent to request a PONG reply. Until then, no "regual" PING was sent,
so Conn_LastPing(<connection>) is null and now becomes non-null in the
PONG command handler.

So the servers are still synchronizing when Conn_LastPing(<connection>)
is 0, which could easily be tested.