* Add a version token on `GetState()`/`SetState()` RPCs for volume server states.
* Make state version a property ov `VolumeServerState` instead of an in-memory counter.
Also extend state atomicity to reads, instead of just writes.
* Boostrap persistent state for volume servers.
This PR implements logic load/save persistent state information for storages
associated with volume servers, and reporting state changes back to masters
via heartbeat messages.
More work ensues!
See https://github.com/seaweedfs/seaweedfs/issues/7977 for details.
* Add volume server RPCs to read and update state flags.