Client side refactoring

-- Introducing a EurekaClientRegistry which feeds to all users of the client.
-- Separated eureka-server, eureka-read-server and eureka-write-server
There is a new module eureka-server which is added and read/write server modules depend on it. Any abstraction common to all servers go in this module and common to client & server goes into eureka-core.
With the current module structure, the eureka-server was dependent on eureka-client which sounds awkward.