Code review comment for lp://staging/~allenap/launchpad/stop-checkwatches-hammering-bug-506158-devel

[The same code has already reviewed in https://code.launchpad.net/~allenap/launchpad/stop-checkwatches-hammering-bug-506158/+merge/17306]

This batches up calls to getModifiedRemoteBugs() so that we don't maim remote trackers. I put some tests in a new file, checkwatches-batching.txt, and have added an XXX to externalbugtracker.txt to suggest that some of the tests in there should be moved to this new file.

The code in _getRemoteIdsToCheck() is still a bit back to front - it should probably identify the new bug watches first, then fill with old ones - but that's beyond the scope of this branch.