Bug #4435: Sync Requests Must Be Deduped

Bug #4435: Sync requests must be deduped to avoid the possibility of building up too many sync requests for the same location

This makes retries a property of the item being monitored rather than the sync request, so even if multiple requests come in, they'll all just be considered retries of the same request, rather than separate requests that each need retry loops of their own.
cRetries?