fix(handler): optimise snapshot hanlding (#8652)

# Which Problems Are Solved

There are cases where not all statements of multiExec are succeed. This
leads to inconsistent states. One example is [LDAP
IDPs](https://github.com/zitadel/zitadel/issues/7959).

If statements get executed only partially this can lead to inconsistent
states or even break projections for objects which might not were
correctly created in a sub table.

This behaviour is possible because we use
[`SAVEPOINTS`](https://www.postgresql.org/docs/current/sql-savepoint.html)
during each statement of a multiExec.

# How the Problems Are Solved

SAVEPOINTS are only created at the beginning of an exec function not
during every execution like before. Additionally `RELEASE` or `ROLLBACK`
of `SAVEPOINTS` are only used when needed.

# Additional Changes

- refactor some unused parameters

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/7959

(cherry picked from commit ddeeeed303)

internal/eventstore/handler/v2/init.go

@@ -264,11 +264,23 @@ func NewViewCheck(selectStmt string, secondaryTables ...*SuffixedTable) *handler
 }
 
 func execNextIfExists(config execConfig, q query, opts []execOption, executeNext bool) func(handler.Executer, string) (bool, error) {
-	return func(handler handler.Executer, name string) (bool, error) {
-		err := exec(config, q, opts)(handler, name)
-		if isErrAlreadyExists(err) {
-			return executeNext, nil
+	return func(handler handler.Executer, name string) (shouldExecuteNext bool, err error) {
+		_, err = handler.Exec("SAVEPOINT exec_stmt")
+		if err != nil {
+			return false, zerrors.ThrowInternal(err, "V2-U1wlz", "create savepoint failed")
 		}
+		defer func() {
+			if err == nil {
+				return
+			}
+
+			if isErrAlreadyExists(err) {
+				_, err = handler.Exec("ROLLBACK TO SAVEPOINT exec_stmt")
+				shouldExecuteNext = executeNext
+				return
+			}
+		}()
+		err = exec(config, q, opts)(handler, name)
 		return false, err
 	}
 }