44663: changefeedccl: use ScanRequest instead of ExportRequest during backfills r=danhhz a=ajwerner

This PR is motivated by the desire to get the memory usage of CDC under
control in the presense of much larger ranges. Currently when a changefeed
decides it needs to do a backfill, it breaks the spans up along range
boundaries and then fetches the data (with some parallelism) for the backfill.

The memory overhead was somewhat bounded by the range size. If we want to make
the range size dramatically larger, the memory usage would become a function of
that new, much larger range size.

Fortunately, we don't have much need for these `ExportRequest`s any more.
Another fascinating revelation of late is that the `ScanResponse` does indeed
include MVCC timestamps (not the we necessarily needed them but it's a good
idea to keep them for compatibility).

The `ScanRequest` permits currently a limit on `NumRows` which this commit
utilized. I wanted to get this change typed in anticipation of #44341 which
will provide a limit on `NumBytes`.

I retained the existing parallelism as ScanRequests with limits are not
parallel.

I would like to do some benchmarking but I feel pretty okay about the
testing we have in place already. @danhhz what do you want to see here?

Relates to #39717.

Release note: None.

44719: sql: add telemetry for uses of alter primary key r=otan a=rohany

Fixes #44716.

This PR adds a telemetry counter for uses
of the alter primary key command.

Release note (sql change): This PR adds collected telemetry
from clusters upon using the alter primary key command.

44721: vendor: bump pebble to 89adc50375ffd11c8e62f46f1a5c320012cffafe r=petermattis a=petermattis

* db: additional tweak to the sstable boundary generation
* db: add memTable.logSeqNum
* db: force flushing of overlapping queued memtables during ingestion
* tool: lsm visualization tool
* db: consistently handle key decoding failure
* cmd/pebble: fix lint for maxOpsPerSec's type inference
* tool: add "find" command
* cmd/pebble: fix --rate flag
* internal/metamorphic: use the strict MemFS and add an operation to reset the DB
* db: make DB.Close() wait for any ongoing deletion of obsolete files
* sstable: encode varints directly into buf in blockWriter.store
* sstable: micro-optimize Writer.addPoint()
* sstable: minor cleanup of Writer/blockWriter
* sstable: micro-optimize blockWriter.store

Fixes #44631 

Release note: None

Co-authored-by: Andrew Werner <ajwerner@cockroachlabs.com>
Co-authored-by: Rohan Yadav <rohany@alumni.cmu.edu>
Co-authored-by: Peter Mattis <petermattis@gmail.com>