* update dbft library, change to 64-bit timestamps and CV reason
* modify messages
* generate stateroot witness data from prepare* messages during commit
Fix#1273.
This seriously improves the serialization/deserialization performance for
several reasons:
* no time spent in `binary` reflection
* no memory allocations being made on every read/write
* uses fast ReadBytes everywhere it's appropriate
It also makes Fixed8 Serializable just for convenience.