sql: preliminary executor logic for savepoints

This commit introduces support for regular SQL savepoints in the
executor.

The following is supported:

- recognizing the SAVEPOINT / ROLLBACK TO SAVEPOINT / RELEASE
  SAVEPOINT statements even when the savepoint name is not
  "cockroach_restart".
- maintaining a list of current active savepoints, and detect
  when SAVEPOINT and RELEASE/ROLLBACK are mismatched.
- issue the `GetSavepoint()` / `RollbackToSavepoint()` /
  `ReleaseSavepoint()` API calls to the KV layers.
- enforce that `cockroarch_restart` (and retry savepoints in general)
  are not started "under" a regular savepoint.

What is not supported yet:

- Rejecting ROLLBACK after a DDL statement has executed successfully.
- ROLLBACK when the txn is aborted (i.e. after an error).

Release note (sql change): CockroachDB now provides
rudimentary (incomplete) and experimental support for SQL savepoints.

Release note (sql change): CockroachDB supports a new statement `SHOW
SAVEPOINT STATUS` which reveals the current stack of active
savepoints. This can be used to teach savepoint usage.

pkg/sql/conn_executor.go

@@ -2407,22 +2407,19 @@ func (sc *StatementCounters) incrementCount(ex *connExecutor, stmt tree.Statemen
 	case *tree.RollbackTransaction:
 		sc.TxnRollbackCount.Inc()
 	case *tree.Savepoint:
-		// TODO(knz): Sanitize this.
-		if err := ex.validateSavepointName(t.Name); err == nil {
+		if ex.isRestartSavepoint(t.Name) {
 			sc.RestartSavepointCount.Inc()
 		} else {
 			sc.SavepointCount.Inc()
 		}
 	case *tree.ReleaseSavepoint:
-		// TODO(knz): Sanitize this.
-		if err := ex.validateSavepointName(t.Savepoint); err == nil {
+		if ex.isRestartSavepoint(t.Savepoint) {
 			sc.ReleaseRestartSavepointCount.Inc()
 		} else {
 			sc.ReleaseSavepointCount.Inc()
 		}
 	case *tree.RollbackToSavepoint:
-		// TODO(knz): Sanitize this.
-		if err := ex.validateSavepointName(t.Savepoint); err == nil {
+		if ex.isRestartSavepoint(t.Savepoint) {
 			sc.RollbackToRestartSavepointCount.Inc()
 		} else {
 			sc.RollbackToSavepointCount.Inc()

pkg/sql/conn_executor_exec.go

@@ -1026,6 +1026,8 @@ func (ex *connExecutor) runObserverStatement(
 	switch sqlStmt := stmt.AST.(type) {
 	case *tree.ShowTransactionStatus:
 		return ex.runShowTransactionState(ctx, res)
+	case *tree.ShowSavepointStatus:
+		return ex.runShowSavepointState(ctx, res)
 	case *tree.ShowSyntax:
 		return ex.runShowSyntax(ctx, sqlStmt.Statement, res)
 	case *tree.SetTracing: