Cleanup and code review for the patch that made bgwriter active during

Cleanup and code review for the patch that made bgwriter active during
archive recovery.  Invent a separate state variable and inquiry function
for XLogInsertAllowed() to clarify some tests and make the management of
writing the end-of-recovery checkpoint less klugy.  Fix several places
that were incorrectly testing InRecovery when they should be looking at
RecoveryInProgress or XLogInsertAllowed (because they will now be executed
in the bgwriter not startup process).  Clarify handling of bad LSNs passed
to XLogFlush during recovery.  Use a spinlock for setting/testing
SharedRecoveryInProgress.  Improve quite a lot of comments.

Heikki and Tom