storage: tweak ScanIntents()

This renames ScanIntents from ScanSeparatedIntents, and adds a
context parameter. Callers have been updated to pass
storage.mvcc.max_intents_per_error for the intent limit, as is done
elsewhere, and targetBytes is passed as 0 for now (no limit) since
other intent collectors currently don't use a byte limit.

Touches #72563.

Release note: None

roachpb: assert request flag combinations

Request flags have implicit dependencies and incompatibilities (e.g.
isLocking implies isTxn). However, these were never checked and
developers were expected to satisfy them manually, which is error-prone.

This patch adds TestFlagCombinations that checks these dependencies
and incompatibilities, based on flagDependencies and flagExclusions
maps which encodes them.

It also adds a new flag type for flags, renames skipLeaseCheck to
skipsLeaseCheck, and adds isAlone for CheckConsistencyRequest.

Release note: None

roachpb: add appliesTSCache request flag

Previously, isIntentWrite determined whether a request checked the
timestamp cache and possibly pushed its timestamp. However, some
requests may want to check the timestamp cache without writing intents,
notably an MVCC-compliant AddSSTable request.

This patch introduces a new flag appliesTSCache, and uses it as a
condition for applying the timestamp cache to the request.

Release note: None

kvserver: add MVCC-compliant AddSSTable variant

AddSSTable does not comply with MVCC, the timestamp cache, nor the
closed timestamp, since the SST MVCC timestamps are written exactly as
given and thus can rewrite history.

This patch adds three new parameters that can make AddSSTable fully
MVCC-compliant, with a corresponding MVCCAddSSTable version gate:

• WriteAtRequestTimestamp: rewrites the MVCC timestamps to the request
  timestamp, complying with the timestamp cache and closed timestamp.

• DisallowConflicts: checks for any conflicting keys and intents at or
  above the SST's MVCC timestamps, complying with MVCC.

• DisallowShadowingBelow: implies DisallowConflicts, and also errors
  if shadowing visible keys (but not tombstones). Unlike the existing
  DisallowShadowing, this allows shadowing existing keys above the given
  timestamp if the new key has the same value as the existing one, and
  also allows idempotent writes at or above the given timestamp.

The existing DisallowShadowing parameter implies DisallowConflicts,
and also errors on any existing visible keys below the SST key's
timestamp (but not tombstones). It no longer allows replacing a
tombstone with a value at the exact same timestamp.

Additionally, even blind AddSSTable requests that do not check for
conflicts now take out lock spans and scan for existing intents,
returning a WriteIntentError to resolve them. This should be cheap
in the common case, since the caller is expected to ensure there are no
concurrent writes over the span, and so there should be no or few
intents.

The WriteAtRequestTimestamp SST rewrite implementation here is correct
but slow. Optimizations will be explored later.

Resolves #70422.

Release note: None