osd: schedule agent from a priority queue

osd: schedule agent from a priority queue

We need to focus agent attention on those PGs that most need it.  For
starters, full PGs need immediate attention so that we can unblock IO.
More generally, fuller ones will give us the best payoff in terms of
evicted data vs effort expended finding candidate objects.

Restructure the agent queue with priorities.  Quantize evict_effort so that
PGs do not jump between priorities too frequently.

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