rework add-mfs to not use caching

[whyrusleeping]

License: MIT
Signed-off-by: Jeromy why@ipfs.io

[ghost]

What's the symlink-related change in this commit?

[hackergrrl]

@lgierth: this PR is a cleaner stab at #2690, which has a bit more history on why symlinks are relevant to this. My understanding is that mfs doesn't support symlinks but since ipfs add uses mfs we need to support symlink caching to get the correct dag back.

[whyrusleeping]

@lgierth its a workaround for the fact that mfs doesnt support symlinks, but we support ipfs add'ing symlinks. You can construct a tree with them in it, but you won't be able to use them via the files api