After the archiver dies, postmaster tries to start a new one immediately.
But previously this could happen only while server was running normally
even though archiving was enabled always (i.e., archive_mode was set to
always). So the archiver running during recovery could not restart soon
after it died. This is an oversight in commit ffd37740.

This commit changes reaper(), postmaster's signal handler to cleanup
after a child process dies, so that it tries to a new archiver even during
recovery if necessary.

Patch by me. Review by Alvaro Herrera.