Apply patch from #961
Apply patch from #961

Author: exarkun
Reviewer: some people looked at it and didn't puke, it is very trivial so I am satisfied with that level of review
Fixes #961

Add an index to grabberID so that the query which is done against it for every
single POP3 UID retrieved by the POP3 grabber actually executes in a reasonable
amount of time.

twisted/quotient,twisted/quotient