Allow ordering by key on mount query

[michaelavila]

This work was done to support https://github.com/ipfs/go-ipfs/pull/6068/files#r266291056.

Overview

Mount datastore used to not support key ordering in queries, now it does.

[Stebalien]

Looking at this change has made me realize that I made a mistake when I said this was going to be really easy. We can have the following mounts:

• /foo
• /foo/bar

In that case, we need to process some of /foo until we hit /foo/bar, then we need to process /foo/bar, then we need to go back to /foo.

There are a bunch of ways to handle this but, IMO, the simplest is to:

1. Query in parallel.
2. Put the next item in each query result (along with a pointer to the query result) into a priority queue.
3. Pull the first item off the priority queue, replacing it with the next item from the query.

This shouldn't be difficult, just slightly more complicated.

[michaelavila]

@Stebalien that makes a lot of sense, we hadn't considered such mounts. I'll work on a change. Thanks.

[michaelavila]

I've added a simplistic and poorly factored solution that addresses the problem, but probably won't be merged as is. A more reasonable solution, I think, is to (1) make the queries all in parallel, and (2) do a k-way merge on the results (which is what I think @Stebalien suggested in IRC).

I'm looking into the better solution now, but I have a proposal on the PR for the original work (https://github.com/ipfs/go-ipfs/pull/6068/files#r267463222) to consider pursuing this as its own task in order to get that other work merged.

[michaelavila]

Superseded by #124