Dont delete sample blocks prematurely (#648)

* Revert "AUP3: Don't delete sample blocks prematurely"

This reverts commit c1884349d5.

* "Don't delete sample blocks prematurely" fixed otherwise...

... and very simply.

Problem was that, only for an interactive effect (like Bass and Treble), the
save point was created, rolled back, created again, then committed.

But (unlike with the non-savepoint commands, even if savepoint is outermost),
rolling back a savepoint really just rewinds it without removing it -- therefore
the second savepoint was inner, but the first (outer) was never committed, so
some changes failed to persist.

Solution:  add a commit after rollback of savepoint to implement destructor
of AutoCommitTransaction.

The reversion of c188434 also leaves AutoCommitTransaction as a better RAII
style operation.  Rollback changes by default -- keep changes only if
success is explicitly indicated.

* Rename AutoCommitTransaction as TransactionScope...

... More appropriately, since it's now the rollback that is automatic but
the commit that must be explicit

src/ProjectFileIO.h

@@ -24,7 +24,7 @@ struct sqlite3_stmt;
 struct sqlite3_value;
 
 class AudacityProject;
-class AutoCommitTransaction;
+class TransactionScope;
 class DBConnection;
 class ProjectSerializer;
 class SqliteSampleBlock;
@@ -240,14 +240,16 @@ private:
 };
 
 // Make a savepoint (a transaction, possibly nested) with the given name;
+// roll it back at destruction time, unless an explicit Commit() happened first.
+// Commit() must not be called again after one successful call.
 // An exception is thrown from the constructor if the transaction cannot open.
-class AutoCommitTransaction
+class TransactionScope
 {
 public:
-   AutoCommitTransaction(ProjectFileIO &projectFileIO, const char *name);
-   ~AutoCommitTransaction();
+   TransactionScope(ProjectFileIO &projectFileIO, const char *name);
+   ~TransactionScope();
 
-   bool Rollback();
+   bool Commit();
 
 private:
    ProjectFileIO &mIO;