Enabling archive_mode without restart

Currently we enable archive_mode only at server start. The reason for
this was to protect against people enabling archive_mode half way
through somebody else running a bulk load without WAL and then having an
incomplete backup.

All we need to do is this:

* When we change archive_mode to "on" get the next xid and place it in
xlogctl. When we turn archive_mode "off" clear the value.

* When we run pg_start_backup() check the xid and then wait for all
transactions to end that can still see that xid before returning.

This means there may be a delay after enabling archive_mode before we
can run a backup, but normal running will not be interrupted.

It's a fairly small change, touching just guc.c and xlog.c, using same
code as DefineIndex() to wait.

Objections?

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support