38211: changefeedccl: remove deprecated poller r=tbg a=danhhz

We switched the default to push-based rangefeeds in 19.1. This removes
the old pull-based poller fallback entirely.

Details of the removal:
- The relevant code is removed
- Several poller-related hacks are removed
- The changefeed.run.push.enabled telemetry metric is removed
- The changefeed.push.enabled cluster setting is removed
- The poller subtest is removed from each changefeedccl test
- The cdc/poller roachtest is skipped on 19.2+
- TestValidations is removed, it's redundant with the much better
  quality TestChangefeedNemeses

Note that the table history still does some polling, but switching this
to RangeFeed will cause an unacceptable increase in the commit-to-emit
latency of rows. This bit of polling will be removed as part of #36289.
This commit also leaves the structure of the changefeed code mostly
unchanged. There is an opportunity for cleanup here, but this also will
wait for after #36289.

Closes #36914

Release note: None

38418: storage: fix null pointer dereference in AdminMerge r=jeffrey-xiao a=jeffrey-xiao

Fixes #38427.

I noticed that `TestRepartitioning` was failing under stress on master with the following error:

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x20894f6]

...

        /usr/lib/go-1.12/src/runtime/panic.go:522 +0x1b5
github.com/cockroachdb/cockroach/pkg/storage.(*Replica).AdminMerge.func1(0xc00441e750, 0xc002a83e60, 0xc00020a880)
        /home/cockroach/go/src/github.com/cockroachdb/cockroach/pkg/storage/replica_command.go:536 +0x226

...
```

The logic for retrieving the RHS descriptor before and after #38302 is not the same in the case where `rightDesc` does not exist. This PR changes it so the behavior is consistent.

It seems possible that the following branch can evaluate true, but I might be missing something here.
```go
// Verify that the two ranges are mergeable.
if !bytes.Equal(origLeftDesc.EndKey, rightDesc.StartKey) {
  // Should never happen, but just in case.
  return errors.Errorf("ranges are not adjacent; %s != %s", origLeftDesc.EndKey, rightDesc.StartKey)
}
```


38435: sql: Fixing interleave check for loose index scan r=rohany a=rohany

Fixing the interleave check for the loose index scan while interleave support is in progress.

Co-authored-by: Daniel Harrison <daniel.harrison@gmail.com>
Co-authored-by: Jeffrey Xiao <jeffrey.xiao1998@gmail.com>
Co-authored-by: Rohan Yadav <rohany@alumni.cmu.edu>