`appyState` will now only read the test session data and update the
environment to relect that state. This is intended for things like
database connections and mailer classes. Actions that need to alter
state to apply it (eg creating a temp db) should be done in other
actions such as 'start'
`saveState` will only take an existing state object and persist it to a
file
Also addresses:
[Connect to test database on session load](https://github.com/silverstripe/silverstripe-testsession/pull/63)
[Skip Temp DB creation if state has no database prop](https://github.com/silverstripe/silverstripe-testsession/pull/64)
Prevent re-creating session file on end (original issue for this PR)
