concurrency: integrate lock modes with SKIP LOCKED

[arulajmani]

Describe the problem

Currently, SKIP LOCKED (via IsKeyLockedByConflictingTxn) performs bespoke conflict resolution, around:

cockroach/pkg/kv/kvserver/concurrency/lock_table.go

Lines 708 to 720 in ea59ee1

	// Key locked.
	txn, ts := l.getLockHolder()
	if strength == lock.None && g.ts.Less(ts) {
	// Non-locking read below lock's timestamp.
	return false, nil
	}
	if g.isSameTxn(txn) {
	// Already locked by this txn.
	return false, nil
	}
	// "If the key is locked, the lock holder is also returned."
	return true, txn
	}

This bakes in the assumption that the lock is of strength Exclusive/Intent. As such, SKIP LOCKED doesn't account for other lock strengths (notably Shared) correctly. We should switch out this function to make use of lock modes instead.

Jira issue: CRDB-30609