mon: Monitor: mon debug cli via admin socket

mon: Monitor: mon debug cli via admin socket

Purpose: allow a dev to test certain portions of the monitors on-the-fly
that would otherwise be nearly impossible without requiring shutting
them off.

Description:

  Currently we are adding support to debug monitors on-the-fly via the
  ceph tool cli, via the admin socket.

  An initial attempt was made to make this interface available via
  over-the-network ceph cli, but that would require a quorum to exist,
  or significant changes to be made to the handling of commands to allow
  this interface to be an exception to the rule.

  Therefore, this interface will live on the admin sockets for the time
  being; potentially forever, because the jury is still out on whether
  there's added value making this available via any other medium that's
  not the admin socket.

  The work means to test the newly added monmap features. This new
  interface will allow one to set, unset and list supported features.

  This will make it easier to implement functional tests for the
  features themselves, for quorum features that are expected to be set
  on-quorum, as well as to test upgrade paths from version A to version
  B.

Signed-off-by: Joao Eduardo Luis <joao@suse.de>