Move PVF code and PoV decompression to PVF host workers (#5142)

Closes #5071 

This PR aims to
* Move all the blocking decompression from the candidate validation
subsystem to the PVF host workers;
* Run the candidate validation subsystem on the non-blocking pool again.

Upsides: no blocking operations in the subsystem's main loop. PVF
throughput is not limited by the ability of the subsystem to decompress
a lot of stuff. Correctness and homogeneity improve, as the artifact
used to be identified by the hash of decompressed code, and now they are
identified by the hash of compressed code, which coincides with the
on-chain `ValidationCodeHash`.

Downsides: the PVF code decompression is now accounted for in the PVF
preparation timeout (be it pre-checking or actual preparation). Taking
into account that the decompression duration is on the order of
milliseconds, and the preparation timeout is on the order of seconds, I
believe it is negligible.