Implement streaming xlog for backup tools

Add option for parallel streaming of the transaction log while a
base backup is running, to get the logfiles before the server has
removed them.

Also add a tool called pg_receivexlog, which streams the transaction
log into files, creating a log archive without having to wait for
segments to complete, thus decreasing the window of data loss without
having to waste space using archive_timeout. This works best in
combination with archive_command - suggested usage docs etc coming later.