walsender parser patch

Attached is an updated version of Heikki's patch to use a parser for
the walsender commands, instead of parsing things manually. It also
does some minor refactoring in walsender.c to break out
IdentifySystem() and StartReplication() to their own functions to make
it more readable.

While having an actual parser here isn't *necessary* at this point, it
makes things easier. And it will become increasingly useful as we add
new features (for example, the "include all wal files" option for
streaming base backup, and I'm sure that sync rep will require some
additional commands or changes to commands).

Any objections to doing this?

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/