changefeedccl: allow users to specify a min_linger on the no-linger sink

[aerfrei]

Follow-up to the no-linger sink. At low-to-moderate throughput the sink
can produce very small batches, increasing the request rate and reducing
the scheduler's ability to build large, conflict-free batches.

Allow users to specify a minimum linger duration. The implementation
should live on the batcher side. One open question is whether batches
need to track the age of the oldest pending key to decide when to flush.

Epic: CRDB-62737

Jira issue: CRDB-64551

[blathers-crl]

cc @cockroachdb/cdc

[jeffswenson]

Here is how I'm thinking about the terminology, a linger duration is how long to wait before flushing a non-full batch. In the same way the linger Messages or linger Bytes are thresholds to flush non-full batches.

The linger batcher will wait for the linger threshold the batch is not full, but will otherwise flush batches as soon as they are full.

The linger duration defaults to zero, which is the "no-linger" mode. The linger Messages or linger Bytes defaults to unlimited. So we typically only flush along time boundaries.