Logout performance

[jhedstrom]

This addresses #62 and includes the logout hardening commit from #393.

[jhedstrom]

To take full advantage of this, the logout method in RawDrupalContext should probably take a $fast parameter (and the method here should be added to a separate interface). Then steps that don't require an actual logout (eg, assertAnonymousUser, etc) can pass in TRUE to utilize the fast logout. This can also be used during user cleanup.

[jhedstrom]

I've decided to merge this and tag an alpha release of 4.0. We can do follow-ups if folks find issues.

[m4olivei]

I think I found an issue with this when using the Selenium Mink Driver, see #446, and corresponding PR #447