This changes the pin behavior. It uses the filenames given through
the api, and allows files to be streamed faltly (not a hierarchy),
which is easier for other things (like vinyl in node-ipfs-api land).
Files can also be entirely out of order, and the garbage intermediate
directories will not be pinned (gc-ed later).

The changes also mean the output of add has changed slightly-- it
no longer shows the local path added, but rather the dag path
relative to the added roots. This is a small difference, but changes
tests.

The dagutils.Editor creates a lot of chaff (intermediate objects)
along the way. Wonder how we might minimize the writes to the
datastore...

This commit also removes the "NilRepo()" part of the --only-hash
mode. We need to store at least in an in-mem repo/datastore because
otherwise the dagutils.Editor breaks.

License: MIT
Signed-off-by: Juan Batiz-Benet <juan@benet.ai>