[docdb] Make TS heartbeat metric reporting to master optional

[bmatican]

Jira Link: DB-2507
The TS currently reports some rocksdb and raft metadata to the master. This should be optional and should not block the server, else we risk jamming the TS. We likely want to have some timed waits on the operations that require rocksdb & raft locks.

We've seen this lead to deadlocks even, for example for restoring data from an encrypted universe:

• the restore blocks rocksdb
• the heartbeater thread calls into rocksdb for stats, so it gets blocked
• the way keys flow back from the master to the TS is as responses to heartbeats

See #4344