(mongodb >=1.4.0)
MongoDB\Driver\Session::getClusterTime — Returns the cluster time for this session
Returns the cluster time for this session. If the session has not been used
for any operation and
MongoDB\Driver\Session::advanceClusterTime() has not
been called, the cluster time will be null.
此函数没有参数。
Returns the cluster time for this session, or null if the session has no
cluster time.