Stop emitting a ValidChunk for every xz stream

[qkaiser]

Very similar to #316

We should return a single ValidChunk holding all the concatenated XZ streams rather than one chunk per stream. Otherwise it leads to situation such as this:

./73479421-74088645.xz_extract
./74088645-74843029.xz_extract
./74843029-74891825.xz_extract
./74891825-75341173.xz_extract
./75341173-75532341.xz_extract
./75532341-75878861.xz_extract
./75878861-76788105.xz_extract
./76788105-77570353.xz_extract
./77570353-78222665.xz_extract
./78222665-79330565.xz_extract