mfs: remove `DAGService` from `Root`
mfs: remove `DAGService` from `Root`

The `Root` structure now explicitly contains a `Directory` (instead of an
`FSNode` interface), use that `Directory`'s `DAGService` instead of its own
`dserv` variable (which was used only once in `closeChild()`). The `DAGService`
in the `Root` and the `Directory` was the same (passed as an argument in the
`NewRoot` initializer function).

This leaves the `Root` structure with only a `Directory` and a `Republisher` and
allows to better rethink its role and whether if those two structures should be
grouped together (and if that group's name should be `Root`).

License: MIT
Signed-off-by: Lucas Molas <715409a5ad5448a3a4a9c37bdefb8f9e1b526eae@gmail.com>

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