[FIX|PERFORMANCE] service discovery was performed in a new thread

For every JID !

Change-Id: I1aea3b73a9573984e0f3c88c6d4b5761ca73d752
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1133
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>