osdc/Objecter: do notify completion callback in fast-dispatch context

osdc/Objecter: do notify completion callback in fast-dispatch context

The notify completion has exactly one user, the librados caller which
does nothing but take a local (inner) lock and signal a Cond.  Do this
in the fast-dispatch context for simplicity.

Notably, this makes the notify completion (and timeout) trigger a
notify2() return (with ETIMEDOUT) even when the finisher queue that
normally delivers notify is busy.. for example with a notify that is
being very slow.  In our case, the unit test is doing a sleep(3) to
test timeouts but also prevented the ETIMEDOUT notification from
being delivered to the caller.  This patch resolves that.

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