Merge pull request #900 from ceph/wip-mon-mds-trim

mon: MDSMonitor: trim versions and let PaxosService decide whether to propose

We were not trimming mdsmap versions and were generating a new map every time
we modified the pending value.

Now we not only make sure that MDSMonitor will trim old maps (configurable
option allowing us to set the maximum number of maps to keep, defaulting to 500,
much like other services do) but we also delegate to PaxosService the decision on
whether to propose our pending value.

We also perform several modifications to 'ceph-kvstore-tool', allowing one to obtain
the contents of a given prefix:key and have them outputted to a file instead of stdout,
and also add support for getting the size of a given prefix:key's value.

'ceph report' was also modified so that we always output the first and last
committed versions for all services; up until this point, we would only output the
first committed version on all services, and only a few were also outputting the
last committed version.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>