[REFACTOR] Rename package net.discoverymanager --> net.discovery

The new package name is more concise and also consistent with the web
page (http://www.saros-project.org/networklayer)

Change-Id: I34a1fe3d981020bc1ccc1fa10869fa2bf25e7ca9
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1460
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Tested-by: Jenkins CI