multiprocessing.Pool.close() ensures no new tasks can be submitted
to the pool and waits for them to all finish. Even though
AsyncResult.get() also waits for the tasks to finish, and our code
structure shouldnt submit new tasks at that point, close() first,
get() then. In the future this could be error-prone in the future
where mp_tasks is modified while results are being merged back and
we miss some results because the iterator wont take these new items
into account *during* iteration.

In Python, the JSON encoder cannot process bytes, the JSON
specification also does not define a "bytes" type. We are
constrained by this in that we cannot serialize data of bytes type.

xattrs can be either strings or bytes, in practice they're likely
representable as strings, therefore, decode as utf-8, error
otherwise. If real world situation of xattrs in true binary format
arise then we will rule out another solution.