This module extends a given voting committee to work across epochs.

This is needed to support the case of validating an old vote or certificate from a previous epoch arriving too late. In the general case, this means we would need to store an arbitrary number of past voting committee selections. However, since: 1. the length of an epoch is much larger than the immutability window, and 2. we don't care about validating votes older than the immutability window, it follows that we only need to store the voting committee selection for the current and previous epochs. NOTE: this rationale might need to be revisited if we ever want to support validating votes and certificates older than the immutability window, e.g., for historical queries.