Database Savepoints (Nested Transactions With Savepoints)

Was implemented functionality: SAVEPOINT (Nested Transactions With Savepoints)

In dialect was added a new methods:
- createSavepoint
- releaseSavepoint
- rollbackSavepoint
- supportsSavepoints
- supportsReleaseSavepoints

In database adapter was added a new methods:
- createSavepoint
- releaseSavepoint
- rollbackSavepoint
- isNestedTransactionsWithSavepoints
- setNestedTransactionsWithSavepoints

Also was changed logic of transactions, so now nested transactions fully supported by database level :)

Was extended Unit-Tests:
- DbTest
- DbDialectTest