Validators' information is completely configured through validators
contracts and does not depend on `authorities.required_signatures`
parameter of bridge's configuration.
The number of validators also could be changed during run-time and
therefore `authorities.required_signatures` parameter will not reflect
the actual number of signatures required for transaction validation.
Solution: retrieve required_signatures from RequiredSignaturesChanged
event and requiredSignatures() method
Closes#74
Yurii Rashkovskii
debris