UserMessage base class, ContactPeersMessage is now a separate UM
added RequestUserMessage abstraction for UMs with a response
minor changes
GetNextFromQueueMessage finished, RequestUserMessage received some
necessary helper methods
ContactPeerUserMessageTest
GetNextFromQueueMessageTest (not working yet)
extended IRequestMessage interface, defined DirectRequestMessage and
RoutedRequestMessage
refactoring of messages and user messages
GetNextUserMessageMessage test re-implemented (not working yet)
further testing, added SerializableLinkedList
Fat bug fix in tests! Try to avoid private classes. They serialize
parent classes references too...
message future listeners renamed
Merge branch 'um-queue'

Conflicts:
	org.hive2hive.core/src/org/hive2hive/core/process/login/SynchronizeFilesStep.java