mon: PaxosService: is_readable/writeable() depending on is_active()

Instead of depending on individual conditions.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: PaxosService: don't use propose_pending directly on request_proposal

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: MDSMonitor: change variable name

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: [MDS]Monitor: remove 'stop_cluster' and 'do_stop()'

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