osd: requeue pg waiters at the front of the finished queue

We could have a sequence like:

- op1
- notify
- op2

in the finished queue.  Op1 gets put on waiting_for_pg, the notify
creates the pg and requeues op1 (and the end), op2 is handled, and
finally op1 is handled.  That breaks ordering; see #2947.

Instead, when we wake up a pg, queue the waiting messages at the front
of the dispatch queue.

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