mon/MDSMonitor: implement new-style cephfs health checks

mon/MDSMonitor: implement new-style cephfs health checks

Our detail elements are still strings, so we keep the bit that collapses
the metadata into a string and appends it to the string.

Each MDS-generated item becomes a detail record.

Health checks are consolidated either by MDS_ or FS_, counting the
number of mds servers or file systems affected.

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