osd: fix agent thread shutdown

We had an old invariant that agent_queue would have at least 1 entry in it to
simplify some other code paths, but it turns out that it is simpler not to do
that.

In particular, this was triggering a failed assertion on shutdown when we 
assert that the queue is empty.

Dump offending items on shutdown if they are there, tho, to catch any future
bugs.

Fixes: #7637 Signed-off-by: Sage Weil <sage@inktank.com>