services: also clear DataFlowInterface in Service::clear()

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
Merge branch 'toolchain-2.7'
Merge branch 'unbuffered-connpolicy' of https://github.com/meyerj/rtt into toolchain-2.7
Merge branch 'service-clear-dataflowinterface' of https://github.com/meyerj/rtt into toolchain-2.7
Merge branch 'global-service-cleanup' of https://github.com/meyerj/rtt into toolchain-2.7
Merge remote-tracking branch 'meyerj/master' into HEAD