Close the session out before erasing all the fixtures.

Not closing the session before closing fixtures results in fatal errors
when using database sessions.

Fixes #3683
Disable the cache when running schema commands.

This prevents stale cached schema definitions from preventing updates.

Fixes #3690
Fix dropTables being ignore in some cases.

When autoFixtures = false & dropTables = false, individually loaded
fixtures should not be dropped.

Fixes #3691
Throw exceptions when non Datasource classes are used as Datasources.

Using models as datasources can cause segmentation faults. Guard against
that by checking types and raising exceptions early.

Fixes #3694
Merge branch 'master' of https://github.com/cakephp/cakephp