Remove in_atomic_block override on the connection, workaround for commit hooks

When implementing the commit hook functionality, I implemented connection.in_atomic_block and made that use our in_atomic_block method.

This confuses Django though, which rightly expects in_atomic_block to return whether we're in a Django atomic(), not a GCloudC atomic().

This MR removes those overrides and comes up with a hacky temporary fix until we can do #10

Closes #11 (closed)