ReplicatedPG: do not ->put() scrub messages when adding to a WorkQueue.

ReplicatedPG: do not ->put() scrub messages when adding to a WorkQueue.

This function is passing a reference from PG::active_rep_scrub to
the req_scrub_wq, not eliminating the reference (and the WorkQueue
doesn't grab a new reference itself, either).
The other alternative is to convert the WorkQueue to grab a
reference, but since they can cycle through the WorkQueue more than
once, and need to be ->put() outside the WorkQueue, I don't like
that option.
This should fix #1758.

Also add an assert to PG::_request_scrub_map to check on the other
possible cause of this bug (and fix the indentation).

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>