Move ReceiveWalConn into its own file. Shuffle constants around so they
are close to the protocol they're associated with, or move them into
postgres_ffi if they seem to be global constants.
Replace XLogRecPtr with Lsn in wal_service.rs .
This removes the last use of XLogSegmentOffset and XLByteToSeg, so
delete them. (replaced by Lsn::segment_offset and Lsn::segment_number.)