remove DeviceRouter interface and implementations

The "routing" step now always returns the plugin associated to the
device.
remove router configuration
shorten import names
remove unused DeviceInfoExtractor implementations
use defaultdict in VotingUpdater
add retriever/updater in RequestProcessing __init__
remove unused DeviceUpdater implementations
add gitignore
remvoe unused ConflictSolver implementations
add ${misc:Pre-Depends} to packages using maintscript

As described in dh_installdeb(1).
remove router.py.conf.default conffile
Merge branch '4974-sync-behind-nat'