mon: MDSMonitor: implement 'get_trim_to()' to let the mon trim mdsmaps

mon: MDSMonitor: implement 'get_trim_to()' to let the mon trim mdsmaps

This commit also adds two options to the MDSMonitor:

  - mon_max_mdsmap_epochs: the maximum amount of maps we'll keep (def: 500)
  - mon_mds_force_trim: the version we want to trim to

This results in 'get_trim_to()' returning the possible values:

  - if we have set mon_mds_force_trim, and this value is greater than the
    last committed version, trim to mon_mds_force_trim
  - if we hold more than the max number of maps, trim to last - max
  - if we have set mon_mds_force_trim and if we hold more than the max
    number of maps, and mon_mds_force_trim is lower than last - max,
    then trim to last - max

Backport: dumpling
Backport: emperor

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>