librados: timeout on mgr_command()

librados: timeout on mgr_command()

because the mgrclient will be waiting for the mgrmap if the mgrmap
is not available, when the client is about to send a mgr command.
and monitor will drop the subscription requests if the client does not
have enough cap for reading mon, so unlike mon commands, the client
won't get an EACCES return code in this case.

in this change, a timeout machinary is introduced. and the client
will wait for "rados-mon-op-timeout" before it gives up. if this
setting is 0, it will wait forever.

Fixes: https://tracker.ceph.com/issues/23627
Signed-off-by: Kefu Chai <kchai@redhat.com>