Generalize engine trait (#6591)

* move common forks and parameters to common params

* port specs over to new format

* fix RPC tests

* parity-machine skeleton

* remove block type

* extract out ethereum-specific methods into EthereumMachine

* beginning to integrate Machine into engines. dealing with stale transitions in Ethash

* initial porting to machine

* move block reward back into engine

* abstract block reward logic

* move last hash and DAO HF logic into machine

* begin making engine function parameters generic

* abstract epoch verifier and ethash block reward logic

* instantiate special ethereummachine for ethash in spec

* optional full verification in verify_block_family

* re-instate tx_filter in a way that works for all engines

* fix warnings

* fix most tests, further generalize engine trait

* uncomment nullengine, get ethcore tests compiling

* fix warnings

* update a bunch of specs

* re-enable engine signer, validator set, and transition handler

* migrate basic_authority engine

* move last hashes into executedblock

* port tendermint

* make all ethcore tests pass

* json-tests compilation

* fix RPC tests: change in gas limit for new block changed PoW hash

* fix minor grumbles

* validate chainspecs

* fix broken import

* fix transaction verification for pre-homestead