Add ability for client to avoid direct deal with base_order

[AlexeyBarabash]

From @bridiver comment https://github.com/brave/sync/pull/226#discussion_r224933820 we should handle the case where all 3 are blank (first node added) - we really have no use or need for the base order in the client and I think we should encapsulate all the logic here so the client doesn't have to know or care about how the order works, so I think we should store it in the lib and use it for this case

Add call setBaseBookmarksOrder(deviceId, platform) which client would call after eash start. Then call getBookmarkOrder(prevOrder, nextOrder, parentOrder) will always succeed even for the case when all thre arguments are empty - this means we want to get order for very root bookmark object (folder) and return baseBookmarksOrder+"1" .