When starting up, SafeBrowsing.jsm will try to use DBService to add testing entries. Meanwhile,
listmanager will request StreamUpdater to download lists with a random initial delay.
The requests that listmanager issue to StreamUpdater will be queued up
if DBserve is busy and will be retried when StreamUpdater is notified that
the previous update is complete. However, in some edge cases,
the queued requests may not be processed until the next update request from listmanager.
For example, SafeBrowsing.jsm calls DBService.beginUpdate at t0 and the update is
complete at t1. If listmanager sends all requests via StreamUpdate between t0 and t1,
they will all be queued up and no further request can trigger the queued ones.
So in this patch I add a timer to re-trigger FetchNextRequest() in case StreamUpdater is not
notified the previous update is complete.
MozReview-Commit-ID: 3hHsS5N7WRI
259a763c10