libceph: support for blacklisting clients

Reuse ceph_mon_generic_request infrastructure for sending monitor
commands.  In particular, add support for 'blacklist add' to prevent
other, non-responsive clients from making further updates.

Signed-off-by: Douglas Fuller <dfuller@redhat.com>
[idryomov@gmail.com: refactor, misc fixes throughout]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>