Epic: none
Release note: None

A Replica comprises a number of spans:

- RangeID-keyed replicated data, such as the `GCThresholdKey`,
  confusingly named "local RangeID keys" since they all have a `/Local/`
(`\x02`) prefix. They have a `r` infix.
- RangeID-keyed unreplicated data, such as the `HardState`,
- The system-internal key-addressable "parallel plane" (such as the
  `RangeDescriptorKey`), confusingly named "local keys" (again, `\x02`
prefix, but `u` infix).
- Two lock spans: one for the "local" keyspace (e.g. for an intent on
  a `RangeDescriptor`), one for the "global" keyspace (e.g. regular
  row intents), in both cases the span is local and uses a `k` infix.
- The actual span of the range (i.e. user data).

Code that needs some or all of these spans gets them from various
methods provided by the `rditer` package. Other than being generally a
messy affair, with a separate raft log, RangeID-based unreplicated keys
will mostly[^1] live on the logstore engine, while RangeID-based
replicated keys will be on the state machine engine and so the current
approach of targeting either only the replicated *or* the combined
(replicated and unreplicated) part of the local RangeID-based span
no longer works. It was also never particularly obvious which spans
one would select with that bool, and I wound up confused more than
once.

The above means we need to stop thinking about RangeID-based keys as
belonging to "one span" - the two RangeID-keyed spans are very different
in nature and storage.

This commit introduces a unified way to select the different parts of the
keyspace. It then goes ahead and uses this new primitive for all span
uses external to the `rditer` package. (`rditer` itself has a few methods
that need exotic combinations of spans, and it gets to do those more
ad-hoc for now but this could be improved later).

In particular, we can now rework `clearRangeData` and give it an
analogoous way to explicitly select which spans to clear. Replica
removal is a central work item in CRDB-220, since it comes up all
over the place: snapshots (in particular in combination with replica
subsumption), splits, replication changes, replicaGC.

After this PR, we can improve splits by not mutating the raft state of
the right-hand side replica in cases where it is known to be newer than
the split. The old API forced us to remove the raft state as well, and
we had to awkwardly lock the right-hand side and write the raft state
back to make things work. We can now simply opt out of mutating that
raft state (via `ClearUnreplicatedByRangeID=false`). This is left for
a separate PR.

This PR makes some additional improvements:

- improve `rditer` tests via `echotest`, also test lock table keys
- improved commentary, esp. around [^1]

[^1]: cockroachdb#95055

Release note: None