* Make mempool aware of MaxGas requirement
* update spec
* Add tests
* Switch GasWanted from kv store to persistent kv store
* Fix typo in test name
* switch back to using kvstore, not persistent kv store
* mempool: Store txs by hash inside of cache
This allows for large cachesizes, without fear of the memory
consumption growing rapidly.
* (squash this) rename hashedTx -> txHash
