Use generics appropriately in RPCEngine and reduce casts, with fixing a related bug of breaking thread-safety in HConnectionManager

In RpcEngine,

  VersionedProtocol getProxy(Class<? extends VersionedProtocol> protocol, ...)

should be

  <T extends VersionedProtocol> T getProxy(Class<T> protocol, ...)

Also, while removing casts I encountered a bug of the method HConnectionManager.HConnectionImplementation.getProtocol() using broken logic just like double-checked locking for HashMap.