DebBugs ExternalBugTracker should have a batch_size of 0

What happens:
At the moment DebBugs, like most other ExternalBugTrackers, has a batch_size of 100, inherited from the ExternalBugTracker base class. This means that only 100 bugs are updated per checkwatches run for checkwatches, which means that DebBugs often falls behind in terms of watches that need checking per day (because there are 11,000 of them).

What should happen
We have a local copy of the DebBugs database, so we should use a batch_size of 0 for DebBugs. This would mean that all the watches that need updating will be updated with each checkwatches run, which should in turn mean that DebBugs won't fall behind any more.