CHIA-1101: update BlockGenerator type

[arvidn]

Purpose:

Simplify and optimize code around block validation.

The BlockGenerator class contains the block generator for a specific block as well as the actual generators for blocks it references.

This PR:

• change the type of generator_refs to List[bytes] instead of List[SerializedProgram]. The only place these items are used are in generators, in CLVM. We always pass plain bytes into CLVM.
• Avoid passing the block generator twice to the worker process for validation. Currently we pass both the FullBLock and the BlockGenerator. By only passing the previous generators (List[bytes]) instead of BlockGenerator, we avoid copying the generator twice.

Current Behavior:

Pass 2 copies of the block generator to pre_validate_blocks_multiprocessing(), once in the FullBlock and once in the BlockGenerator

New Behavior:

Only pass the block generator once to pre_validate_blocks_multiprocessing(), in the FullBlock. Instead of the BlockGenerator just pass the generator_refs.

[arvidn]

The performance improvement is found in chia/consensus/multiprocess_validation.py. Otherwise there's a mixture of updates to tests with some more simplifications in chia/full_node/mempool_check_conditions.py

[wjblanke]

aok