We were completely lacking ValidatorsCount that is supposed to track the
number of votes with particular count of consensus nodes which in theory can
change the number of active consensus nodes (if it ever to exceed the number
of standby validators), so we were not producing the right count and based on
that not giving the right set of validators.
Fixes#512.
Simple as that: UnregisteredAndHasNoVotes != !RegisteredAndHasVotes
Registered validators should stay in the DB, we might be in the process of
updating votes for them and that starts with subtraction.
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.
