changefeedccl: re-examine how checkpoints are affected by ALTER CHANGEFEED

[andyyang890]

I came across these two pieces of code to generate a new checkpoint when we alter a changefeed that only assigns spans without a timestamp:

cockroach/pkg/ccl/changefeedccl/alter_changefeed_stmt.go

Lines 802 to 804 in f06be00

	Checkpoint: &jobspb.ChangefeedProgress_Checkpoint{
	Spans: existingTargetSpans,
	},

cockroach/pkg/ccl/changefeedccl/alter_changefeed_stmt.go

Lines 832 to 834 in f06be00

	Checkpoint: &jobspb.ChangefeedProgress_Checkpoint{
	Spans: mergedSpanGroup.Slice(),
	},

It seems to me that this checkpoint would not only not help restore progress of those spans, but actually regress the progress of those spans back to zero given we assume that the checkpoint is always greater than the highwater mark:

cockroach/pkg/ccl/changefeedccl/changefeed_dist.go

Lines 472 to 474 in 9eb1027

	if checkpointSpanGroup.Encloses(nodeSpan) {
	initialResolved = checkpoint.Timestamp
	}

We should re-examine what the expected behavior should be and fix the code to reflect that.

Jira issue: CRDB-45815

[blathers-crl]

Hi @andyyang890, please add branch-* labels to identify which branch(es) this C-bug affects.

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[blathers-crl]

cc @cockroachdb/cdc