mon: make 'health' warn about slow requests

Currently we see slow request warnings go by in the cluster log, but they
are not reflected by 'ceph health'.  Use the new op queue histograms to
raise a flag there as well.

For example:

HEALTH_WARN 59 requests are blocked > 32 sec; 2 osds have slow requests
21 ops are blocked > 65.536 sec
38 ops are blocked > 32.768 sec
16 ops are blocked > 65.536 sec on osd.1
23 ops are blocked > 32.768 sec on osd.1
5 ops are blocked > 65.536 sec on osd.2
15 ops are blocked > 32.768 sec on osd.2
2 osds have slow requests

Fixes: #5505
Signed-off-by: Sage Weil <sage@inktank.com>