Dummy Channel - Registry

[simongregorebner]

Currently the ChannelService object returns different Dummy Channels although the name of the dummy channel is the same (in the descriptor).
This is very bad for testing and does not comply to the ChannelAccess idea.

Therefor there need to be a map of already created channels which is searched first before a new dummy channel is created and send back. If an entry in the map exist the create function should just return this entry.

clear the map once the service is stopped/destroyed.

Problems to expect:
Request for the same channel but of a different type.