cleanup old sreqs if the master changed
cleanup old sreqs if the master changed

Jumping back and forth between different masters will over time add more
and more sreqs to the sreq_cache. That might be ok in MultiMaster, with
signed master-pubkeys, we only want one single master sreq we re-use and
remove all the others.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt