This commit adds support for rolling a transaction back across a
`LockNotAvailable` (pgcode 55P03) error. `LockNotAvailable` errors
are returned in two cases:
1. when a locking SELECT is run with a NOWAIT wait policy and conflicts
   with an active lock
2. when a statement is run with a `lock_timeout` and this timeout is
   exceeded (unsupported, see cockroachdb#67513)

The following test case from `pkg/sql/testdata/savepoints` demonstrates
this new capability:

```
// txn1
BEGIN
INSERT INTO t VALUES (1)
----
1: BEGIN -- 0 rows
-- NoTxn       -> Open        #.  (none)
2: INSERT INTO t VALUES (1) -- 1 row
-- Open        -> Open        ##  (none)

// txn2
BEGIN
SAVEPOINT foo
SELECT * FROM t WHERE x = 1 FOR UPDATE NOWAIT
ROLLBACK TO SAVEPOINT foo
SELECT * FROM t WHERE x = 2 FOR UPDATE NOWAIT
COMMIT
----
1: BEGIN -- 0 rows
-- NoTxn       -> Open        #.....  (none)
2: SAVEPOINT foo -- 0 rows
-- Open        -> Open        ##....  foo
3: SELECT * FROM t WHERE x = 1 FOR UPDATE NOWAIT -- pq: could not obtain lock on row (x)=(1) in t@primary
-- Open        -> Aborted     XXXXXX  foo
4: ROLLBACK TO SAVEPOINT foo -- 0 rows
-- Aborted     -> Open        ##....  foo
5: SELECT * FROM t WHERE x = 2 FOR UPDATE NOWAIT -- 0 rows
-- Open        -> Open        ##..#.  foo
6: COMMIT -- 0 rows
-- Open        -> NoTxn       ##..##  (none)
```

This becomes the second error type that supports rollbacks, with the
first being duplicate key errors, which was added in 65e8045.

The primary motivation for this PR was to be able to give
`WriteIntentErrors` an `ErrorPriority` of `ErrorScoreUnambiguousError`.
However, the added functionality fell out of making that change.

Release note (sql change): ROLLBACK TO SAVEPOINT can now be used to
recover from LockNotAvailable errors (pgcode 55P03), which are returned
when performing a FOR UPDATE SELECT with a NOWAIT wait policy.