Add current WAL end (as seen by walsender, ie, GetWriteRecPtr() result)

Add current WAL end (as seen by walsender, ie, GetWriteRecPtr() result)
and current server clock time to SR data messages.  These are not currently
used on the slave side but seem likely to be useful in future, and it'd be
better not to change the SR protocol after release.  Per discussion.
Also do some minor code review and cleanup on walsender.c, and improve the
protocol documentation.