[API] Adds core.concurrent package

Change-Id: I56ecf50a8151b454060c7165e417da3b7c346b07
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1882
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Tested-by: Jenkins CI
Reviewed-by: Holger Schmeisky <google@schmeisky.com>