Improve reliability of sync when devices are connected concurrently

[bsclifton]

Per comments by @ayumi in https://github.com/brave/sync/issues/112

I found that:

1. if both pyramids are online concurrently, there might be a delay in what pyramid 1 sees from the data store (S3). this can cause bookmarks to fail to download.
2. if pyramid 0 syncs, then you wait 10 minutes, then pyramid 1 syncs, there is much less loss of bookmarks. MacOS, browser-laptop 0.17.x

AWS S3 "eventual consistency"

Ideas to make this better:

1. Browser-clients should reupload pending sync records after time t (currently reupload happens on browser restart).
2. Browser-clients concurrently online should detect S3 inconsistency (maybe by periodically measuring total record count) and redownload all records when that happens.