add infrastructure to receive higher order commands, like remove_device

add infrastructure to receive higher order commands, like remove_device

From: Dan Williams <dan.j.williams@intel.com>

Each md_message encapsulates a single command.  A command includes an 'action'
member which describes what if any data comes after the action.  Communication
with the monitor involves updating the active_cmd pointer and then writing to
mgr_pipe.  Pass/fail status is returned via mon_pipe.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>