YARN-913: with separate ZK client and server contexts, ZK as client can write to server of ZK-the-service
YARN-913 anonymous access correctly disables SASL client access operations
YARN-913 RegistrySecurity does not attempt to set ZK security settings if the registry is not secure
YARN-913 whitespace, javadocs and formatting
YARN-913 specific constants for ALICE_CLIENT_CONTEXT and BOB_CLIENT_CONTEXT  in secure tests
YARN-913.
1. factory takes a client context when setting up the kerberos context. This is (currently) explicit.
2. TestSecureRMRegistryOperations failing when trying to log in Alice; usual kerberos checksum problem again
YARN-913 Add actions to add a new write accessor to a path, and to reset them
YARN-913  fix invalid default set of system accounts
YARN-913 curator service checks state before core zK actions, so fails meaningfully if called before the service is started
YARN-913 better diags
YARN-913 rename RegistryOperationsFactory method
YARN-913 ongoing work on secure tests
YARN-913 digest auth is working for setting permissions and for client auth
YARN-913 digest auth tests for missing ID/password in factory and lower down
YARN-913 tests all working; current user is only added as a sasl: user if a secure user; anon accessors must be given a real account