mds: support limiting cache by memory

mds: support limiting cache by memory

This introduces two config parameters:

    mds_cache_memory_limit: Sets the soft maximum of the cache to the given
    byte count. (Like mds_cache_size, this doesn't actually limit the maximum
    size of the cache. It just dictates the steady-state size.)

    mds_cache_reservation: This replaces mds_health_cache_threshold everywhere
    except the Beacon heartbeat sent to the mons. The idea here is to specify a
    reservation of memory (5% by default) for operations and the MDS tries to
    always maintain that reservation. So, the MDS will recall caps from clients
    when it begins dipping into its reservation of memory.

mds_cache_size still limits the cache by Inode count but is now by-default 0
(i.e. unlimited). The new preferred way of specifying cache limits is by memory
size. The default is 1GB.

Fixes: http://tracker.ceph.com/issues/20594
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1464976
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>