A more comprehensive model for PoV-Blocks and Candidate receipts (#843)

* encode the candidate statement as only the hash

* refactor CandidateReceipt and CollationInfo

* introduce an abridged candidate receipt type

* erasure coding stores candidate receipt

* store omitted data instead and introduce AvailableData type

* refactor availability-store schema

* tweak schema and APIs a bit more

* get availability-store tests passing

* accept AbridgedCandidateReceipt in `set_heads`

* change statement type in primitives to be hash-only

* fix parachains runtime tests

* fix bad merge

* rewrite validation pipeline

* remove evaluation module

* use abridged candidate hash as canonical

* statement table uses abridged candidate receipts

* kill availability_store::Data struct

* port shared table to new validation pipelines

* extract full validation pipeline to helper

* remove old validation pipeline from collation module

* polkadot-validation compiles

* polkadot-validation tests compile

* make local collation available in validation service

* port legacy network code

* polkadot-network fully ported

* network: ensure fresh statement is propagated

* remove pov_block_hash from LocalValidationData

* remove candidate_hash field from AttestedCandidate and update runtime

* port runtimes to new ParachainHost definition

* port over polkadot-collator

* fix test compilation

* better fix

* remove unrelated validation work dispatch fix

* address grumbles

* fix equality check