Lock controller during fetching

[albireox]

During fetching we often see a TIMEOUT status that causes the controller to go into ERROR state, although the fetching completes normally.

The TIMEOUT seems caused by one of the STATUS or SYSTEM commands that are issued all the time by cerebro and others. In b53f8cd I prevented these timeouts from setting the ERROR bit in ControllerStatus, but there should be a better solution. Probably this mostly happens during fetching because it's the most CPU intensive operation for the Archon. We can add a lock during fetching so that status and system commands are prevented to run until fetching completes. Fetching doesn't call those commands so it should be fine.