…geController

This is used to set the highest index up to which all send-queues in
pull mode must be force-flushed.

Informs cockroachdb#135601

Epic: CRDB-37515

Release note: None

- ReplicatedEvalResult.DoTimelyApplicationToAllReplicas is added and set
  in splitTriggerHelper, MergeTrigger, Migrate, Subsume.
- The previous setting to true is gated on cluster version
  V25_1_AddRangeForceFlushKey.
- This causes ReplicaState.ForceFlushIndex to be set, and
  RangeForceFlushKey (a replicated range-id local key) to be written when
  applying the corresponding batch to the state machine. The index is set
  to the index of the entry being applied, and is monotonically
  increasing.
- replica_rac2.Processor and rac2.RangeController have
  existing ForceFlushIndexChangedLocked methods that are called whenever
  the Replica sees a change in the force-flush-index.

Fixes cockroachdb#135601

Epic: CRDB-37515

Release note: None

Add a new rac2 flow control integration test,
`TestFlowControlSendQueueRangeSplitMerge`.

This test takes the following steps:

```sql
-- We will exhaust the tokens across all streams while admission is blocked on
-- n3, using a single 4 MiB (deduction, the write itself is small) write. Then,
-- we will write a 1 MiB put to the range, split it, write a 1 MiB put to the
-- RHS range, merge the ranges, and write a 1 MiB put to the merged range. We
-- expect that at each stage where a send queue develops n1->s3, the send queue
-- will be flushed by the range merge and range split range operations.
```

Part of: cockroachdb#132614
Release note: None