coreapi/unixfs: in /add, don't setup a MFS root with a mock

[MichaelMure]

It seems to me that this section of the code is unnecessary as a similar setup would be done in the adder already.

Additionally that would avoid using a test mock, which is questionable to begin with.

[MichaelMure]

Back to draft as this fails the CI. My assumption was obviously wrong, but I'll use that PR to discuss.

If I get that right:

• with --only-hash, we still want to simulate what would happen with MFS, I assume to return the proper hash
• to do so, we need a MFS with a root dir around
• the problem with the only-hash code path is that we push blocks to null datastore, so there is no MFS root stored there
• to workaround the problem, another DagService is created with an in-memory datastore, specifically to store that empty MFS root, to be pulled later in the adder

Now, I don't understand why we do this ? Why not just wrap in an empty unixfsdir? Why even wrap?

Would there be a way to simplify that code?