mon: clear pg delta after some period

If we have not pg_map updates, the delta doesn't update, and can get stuck
with the velocity right before activity stopped.  This is confusing, and
can cause incorrect health warnings about in-progress recovery.

To fix this, zero the delta if there is no activity for
'mon delta reset interval' seconds.

Fixes: #5077
Signed-off-by: Sage Weil <sage@inktank.com>