Add explicit DB transaction management

* Added methods on DB API for explicit transaction demarcation
* Added @session_aware() decorator maker
* Refactoring SqlAlchemy implementation of DB API
* Added all tests for DB API
* Added methods to access thread local variables in mistral/utils.py
* Fixed API Resource class to ignore unset values in transformations

Change-Id: I5fae196f89c77a5e355207907bd08983969922c3