Add a new role for ratekeeper.
Remove StorageServerChanges from data distribution.
Ratekeeper monitors storage servers, which borrows the idea from
DataDistribution.
Use getRateInfo's endpoint as the ID for the DataDistributorInterface.
For now, added a "rejoined" flag for ClusterControllerData and Proxy.
TODO: move DataDistributorInterface into ServerDBInfo.
Let cluster controller to start a new data distributor role by sending a
message to a chosen worker.
Change MasterInterface usage in DataDistribution to masterId
Add DataDistributor rejoin handling.
This allows the data distributor to tell the new cluster controller of its
existence so that the controller doesn't spawn a new one. I.e., there should
be only ONE data distributor in the cluster.
If DataDistributor (DD) doesn't join in a while, then ClusterController (CC) tries
to recruit one as DD. CC also monitors DD and restarts one if it failed.
The Proxy is also monitoring the DD. If DD failed, the Proxy will ask CC for
the new DD.
Add GetRecoveryInfo RPC to master server, which is called by data distributor
to obtain the recovery Transaction version from the master server.
Jingyu Zhou