Count consensus messages and change to prepare state handling commit

[aidan-kwon]

Istanbul BFT could fail to achieve a consensus on a hash-locked proposal depends on the consensus message order. With this change, the problem can be resolved. The detail about the problem is described in https://github.com/ConsenSys/quorum/issues/1133

Resolve https://github.com/ConsenSys/quorum/issues/1133

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[antonydenyer]

Closing as should be solved with QBFT