CHIA-1291 Switch mempool TX prevalidation to the Rust version

[AmineKhaldi]

Purpose:

Leverage the Rust version of validate_clvm_and_signature with the added benefit of being able to use ThreadPools instead of ProcessPools.

Current Behavior:

Mempool item prevalidation currently happens with a Python version of validate_clvm_and_signature through ProcessPools.

New Behavior:

Mempool item prevalidation happens with a Rust version of validate_clvm_and_signature through ThreadPools.

[arvidn]

Missing coverage:

chia/full_node/mempool_manager.py (91.7%): Missing lines 288
chia/types/eligible_coin_spends.py (54.5%): Missing lines 342-345,347

Are these OK Amine?

[AmineKhaldi]

Missing coverage:

chia/full_node/mempool_manager.py (91.7%): Missing lines 288
chia/types/eligible_coin_spends.py (54.5%): Missing lines 342-345,347

Are these OK Amine?

Yes:

• For
  chia/full_node/mempool_manager.py:288 and chia/types/eligible_coin_spends.py:347 these are just defensive. I documented the failure path and marked these with the no cover pragma.

• For chia/types/eligible_coin_spends.py:342-345 it's existing logic, and I didn't mark it with the no cover pragma because this can technically happen, we just don't currently have a realistic test scenario that we can add, where a singleton would be somehow broken after it gets fast forwarded. This is therefore fine by me to continue exempting.