Add support for nested transactions

[smvv]

Setting the flag nested=True on the transaction hook allows nested sqlalchemy transactions.

Instead of creating and destroying all SQL tables before/after a unit test, it is faster to start a transaction and perform a rollback after the unit test. The problem is that without the nested=True flag, the transaction hook will commit the changes in the on_response method. When rolling the transaction back, the transaction is already committed and that causes the rollback to fail.

Another problem is that the transaction hook will remove the session, which will commit the started transaction outside of the transaction hook. When setting nested=True, it is required that the session is removed during the unit test's setup_method using database.Session.remove(). Otherwise, sqlalchemy will report a warning about a session scope that is already in use.

edit: Appearently, python 3.5 fails with sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such savepoint: sa_savepoint_2 [SQL: 'RELEASE SAVEPOINT sa_savepoint_2']. Sqlite in Python 3.5 has a default setting that doesn't work as expected and that setting has been changed in python 3.6: https://docs.python.org/3/library/sqlite3.html#sqlite3-controlling-transactions.

See also the related Python stdlib issue: https://bugs.python.org/issue10740.

[codecov]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #2   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          40     48    +8     
  Branches        1      4    +3     
=====================================
+ Hits           40     48    +8

Impacted Files	Coverage Δ
apistar_sqlalchemy/components.py	100% <100%> (ø)	⬆️
apistar_sqlalchemy/event_hooks.py	100% <100%> (ø)	⬆️

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[perdy]

I'm happy with the changes but I'd like to keep the tests at a higher level. Maybe can you modify the current test_end_to_end.py file to test plain and nested transactions behavior? In that case we wouldn't need to extract the model into a different file.

Thanks for that !