improve pg_receivewal code

Hi,

I see there's a scope to do following improvements to pg_receivewal.c:

1) Fetch the server system identifier in the first RunIdentifySystem
call and use it to identify(via pg_receivewal's ReceiveXlogStream) any
unexpected changes that may happen in the server while pg_receivewal
is connected to it. This can be helpful in scenarios when
pg_receivewal tries to reconnect to the server (see the code around
pg_usleep with RECONNECT_SLEEP_TIME) but something unexpected has
happnend in the server that changed the its system identifier. Once
the pg_receivewal establishes the conenction to server again, then the
ReceiveXlogStream has a code chunk to compare the system identifier
that we received in the initial connection.
2) Move the RunIdentifySystem to identify timeline id and start LSN
from the server only if the pg_receivewal failed to get them from
FindStreamingStart. This way, an extra IDENTIFY_SYSTEM command is
avoided.
3) Place the "replication connetion shouldn't have any database name
associated" error check right after RunIdentifySystem so that we can
avoid fetching wal segment size with RetrieveWalSegSize if at all we
were to fail with that error. This change is similar to what
pg_recvlogical.c does.
4) Move the RetrieveWalSegSize to just before pg_receivewal.c enters
main loop to get the wal from the server. This avoids an unnecessary
query for pg_receivewal with "--create-slot" or "--drop-slot".
5) Have an assertion after the pg_receivewal done a good amount of
work to find start timeline and LSN might be helpful:
Assert(stream.timeline != 0 && stream.startpos != InvalidXLogRecPtr);

Attaching a patch that does take care of above improvements. Thoughts?

Regards,
Bharath Rupireddy.