WAL "low watermark" during base backup

Attached patch implements a "low watermark wal location" in the
walsender shmem array. Setting this value in a walsender prevents
transaction log removal prior to this point - similar to how
wal_keep_segments work, except with an absolute number rather than
relative. For now, this is set when running a base backup with WAL
included - to prevent the required WAL to be recycled away while the
backup is running, without having to guestimate the value for
wal_keep_segments. (There could be other ways added to set it in the
future, but that's the only one I've done for now)

It obviously needs some documentation updates as well, but I wanted to
get some comments on the way it's done before I work on those.

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