DRILL-4280: CORE (user to bit authentication, Java)
+ Add logic for authentication in UserClient and UserServer with
backward compatibility in both directions
+ Add abstract extension to ServerConnection and ClientConnection
+ Add concrete extensions to abstract connections:
BitToUserConnection and UserToBitConnection
+ Add ConnectionConfig interface with abstract and concrete
implementations to encapsulate configuration for server-side
connections
+ Encapsulate all requests handled by UserServer in
UserServerRequestHandler
+ Clear UserSession when connection is closed either by user or
bit
+ Add DrillProperties to encapsulate all connection properties
used during connection time
