A fix for Bug#955222 "Replication relay picks up .inprogress file"

Replication relay would actually *not* pick up .inprogress file,
and would continue looking for .xlog file every wal_dir_rescan_delay
seconds, and print errors in the error log.

As a side effect, the last record could be not replicated
for a long time if a master doesn't have any updates after
creating an .xlog file.

The fix is, when doing the recovery for replication relay (or for
local hot standby, or otherwise), is to look for .inprogress
file first, if it is found, use it/rename it.

No test case since so far we can't grep the error log for
things that should not occur in it.