mon/MonClient: don't return zero global_id

mon/MonClient: don't return zero global_id

active_con is reset after re-connecting. get_global_id() return 0
before mon client receives the MAuthReply. The zero global_id
confuses mds code. in this change, MonClient promote the active_con's
global_id to this->global_id once it's authorized. and returns
this->global_id in get_global_id().

Fixes: http://tracker.ceph.com/issues/19134
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>