mds/MDBalancer: update MDSRank export_targets according to current balance state
I think this might be a regression issue introduced by https://github.com/ceph/ceph/commit/082e86c58f5abb93e2a912e603d86f12bc47972c. We didn't see such issue in Jewel when trying to use old balancer.
The reason should be that currently MDS's export_targets in MDSMap are empty even if they need to do rebalance because we didn't update MDSRank's export_targets according to balance state, then MDSRank's tick won't update MDSMap's export_targets to Mon.
And the new function MDBalancer::hit_targets should be aimed to do such things, but it hadn't been called at anywhere.