Decide on block hashing specifics

Today, the orderer is using the functions defined in `fabric/protos/common/block.go` for hashing the block header and data.

These functions are definitely wrong, and were never intended to be long term solutions. These functions need to be fixed to use a hashing algorithm and marshalling scheme (and possibly using a wide Merkle Tree for the BlockData).

This was discussed somewhat extensively in https://gerrit.hyperledger.org/r/#/c/1361/ but no conclusion was reached.