task/mds_thrash: Log mds dump after long delay

In cases where the mds thrasher continuously loops
waiting for an mds to be removed from the map, or
for a new mds to become active, we want to start logging
the mds state for debugging.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Merge pull request #6 from ceph/wip-mds-thrasher-logging

task/mds_thrash: Log mds dump after long delay