31013: kv: try next replica on RangeNotFoundError r=nvanbenschoten,bdarnell a=tschottdorf

Previously, if a Batch RPC came back with a RangeNotFoundError, we would
immediately stop trying to send to more replicas, evict the range
descriptor, and start a new attempt after a back-off.

This new attempt could end up using the same replica, so if the
RangeNotFoundError persisted for some amount of time, so would the
unsuccessful retries for requests to it as DistSender doesn't aggressively
shuffle the replicas.

It turns out that there are such situations, and the election-after-restart
roachtest spuriously hit one of them:

1. new replica receives a preemptive snapshot and the ConfChange
2. cluster restarts
3. now the new replica is in this state until the range wakes
   up, which may not happen for some time. 4. the first request to the range
   runs into the above problem

@nvanbenschoten: I think there is an issue to be filed about the tendency
of DistSender to get stuck in unfortunate configurations.

Fixes #30613.

Release note (bug fix): Avoid repeatedly trying a replica that was found to
be in the process of being added.

31187: roachtest: add synctest r=bdarnell a=tschottdorf

This new roachtest sets up a charybdefs on a single (Ubuntu) node and runs
the `synctest` cli command against a nemesis that injects random I/O
errors.

The synctest command is new. It simulates a Raft log and can be directed at a
filesystem that is being hit with random failures.

The workload essentially writes ascending keys (flushing each one to disk
synchronously) until an I/O error occurs, at which point it re-opens the
instance to verify that all persisted writes are still there. If the
RocksDB instance was permanently corrupted, it switches to a new, pristine,
directory.
This is used in the roachtest, but is also useful for manual use in user
deployments in which we suspect there is a failure to persist data to disk.

This hasn't found anything, but it's fun to watch and also shows us a
number of errors that we know and love from sentry.

Release note: None

31215: storage: deflake TestStoreRangeMergeWatcher r=tschottdorf a=benesch

This test could deadlock if the LHS replica on store2 was shut down
before it processed the split at "b". Teach the test to wait for the LHS
replica on store2 to process the split before blocking Raft traffic to
it.

Fixes #31096.
Fixes #31149.
Fixes #31160.
Fixes #31167.

Release note: None

31217: importccl: add explicit default to mysql testdata timestamp r=dt a=dt

this makes the testdata work on mysql 8.0.2+, where the timestamp type no longer has the implicit defaults.

Release note: none.

31221: cluster: Create final cluster version for 2.1 r=bdarnell a=bdarnell

Release note: None

Co-authored-by: Tobias Schottdorf <tobias.schottdorf@gmail.com>
Co-authored-by: Nikhil Benesch <nikhil.benesch@gmail.com>
Co-authored-by: David Taylor <tinystatemachine@gmail.com>
Co-authored-by: Ben Darnell <ben@bendarnell.com>