Improvements to SASL

Points that occur to me right now:

1. The login object in ZooKeeperSaslClient is static, which means that if you try to create another client for tests, the login object will be the first one you've set for all runs. I've experienced this with 3.4.6.
2. There are a number of properties spread across the code that do not appear in the docs. For example, zookeeper.allowSaslFailedClients isn't documented afaict.