MB-7592 Race condition free management of tmp files

Don't delete the temporary files when the compactor is
started or when it finishes because the updater might
still be using some of them.
Also, whenever the compactor is shutdown or dies, ensure
the log files field is reset.

Change-Id: I89209b4cffff1cad42f190dd465cbbd83e5325af
Reviewed-on: http://review.couchbase.org/24178
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>