Consider extracting validation code (aka PVF) handling from paras into a separate module

[pepyakin]

Paras module now is burdened with many responsibilities:

1. Keeps track what paras are registered and which are parathreads. Can answer queries about those.
2. Handles the lifecycle changes of a parachain: onboarding/offboarding as well as migration between parachain and parathread states.
3. Keeps track the latest head data.
4. Manages validation code. Specifically,
   1. Code repository, which basically stores validation codes on chain and performs reference counting on validation code preimages.
   2. Maintains a mapping of the current, past and future PVFs for each parachain, also allowing queries about it.
   3. Manages the upgrade sequence. Reject blocks if they attempt to perform a not pertinent upgrade. Applies the upgrades when it's time.
5. After Tracking issue: Pre-checking for PVF Compilation time polkadot#3211, it will also include the logic required for pre-checking, specifically:
   1. Runtime API for returning the validation code hashes that require pre-checking.
   2. Accepting unsigned transactions with votes for either validity or invalidity of a PVF in question.
   3. Handling tallying on session changes.
6. After runtime upgrade: Methods to avoid ever including parachain code in critical-path data #967, it will also include the logic for:
   1. Waiting for the validation code preimages.
   2. Accepting unsigned transactions with validation code preimages.

Looking at this, one may think that validation code handling is a separate concern, which is not tied to the other things paras is doing. Based on that observation, it may be a good idea to introduce a module that will take care of the things related to validation code.