mon: MonClient: isolate ping_monitor() from monc

By not using monc's member variables, we can reuse the client's monc's
instance without causing havoc in the setup.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
librados: RadosClient: reuse already setup monc during ping_monitor()

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: MonClient: make input parameters const

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: MonClient: document ping_monitor()

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
include: rados: librados: document rados_ping_monitor()

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