Reimplement assign/unassign related procedure metrics

Now we have four types of RIT procedure metrics, assign, unassign, move, reopen. The meaning of assign/unassign is changed, as we will not increase the unassign metric and then the assign metric when moving a region. Also introduced two new procedure metrics, open and close, which are used to track the open/close region calls to region server. We may send open/close multiple times to finish a RIT since we may retry multiple times.