It seemed like a good idea to decompress xz-compressed files before
handing over decompressed bytes to descriptor implementations.
However, our web logs expect descriptor bytes to be in compressed
form, and now they're unhappy about receiving uncompressed bytes.
Changing this back and teaching OnionPerf analysis files to expect
compressed bytes, too. Found by running CollecTor's unit tests.

Implements #34070.

Implements #33549.