Merging the engine server inside the engine repository

See https://bugs.launchpad.net/oq-engine/+bug/1284681 for the motivations.

This PR only merge the existing code; some changes to packager.sh and run_tests.sh will be needed; moreover there is a dependency on concurrent.futures to address (see http://ci.openquake.org/job/zdevel_oq-engine/301/)