Change method name in HelloMessage

HelloMessage used getUUID() to return the UUID of the remote module
thereby shadowing getUUID() of Message (which returns the message UUID).
The method in HelloMessage is now called getModuleUUID()
Add UniqueUUIDUtil

Provides an easy way to generate UUIDs that are guaranteed to be different.
This adds some overhead and randomly generating two equal UUIDs is almost
impossible, so this is not used in production use, only for tests.
Fix MockChannel attributes

Attributes were not set correctly on MockChannel