[Segment Replication] Update components of segrep backpressure to support remote store.

[mch2]

Currently, Segment Replication pressure is enforced using a count of checkpoints and time a replica is behind its primary. This data is kept in the primary's ReplicationTracker, given that writes are rejected at the primary, and updated from SegmentReplicationSource service when a primary has finished sending segments for a particular checkpoint to a replica.

With remote storage as the source of replication, this will have to change to return necessary data the no-op write path or perform an async poll of replica state so primaries can perform its bookkeeping.

[ankitkala]

I'll pick it up.