fix(core): allow authentification in RO and RW modes
fix(core): allow authentification in RO and RW modes

Fixes the bug introduced by PR #512 where it is not possible to
connect in RO mode using authentication. Since the SASL
authentification feature, the _session_fallback method should
be called only after the SASL authentification is done. A new
method is added to the ConnectionHandler class in order to
correctly use KeeperState.CONNECTED or KeeperState.CONNECTED_RO.

python-zk/kazoo,python-zk/kazoo