[API] networklayer can now establish multiple connections

The networklayer can now establish multiple connections. Each
connection now requires an ID. Added a new sendData method
which no longer auto connects on demand to ensure that broken
connection do not get "lost".

Change-Id: I115c3de96533724c29d83c0fe7a7d81d1b5b67d0
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/959
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>