Create a `RosterHistory` class able to return current and previous rosters and lookup by round number.

[edward-swirldslabs]

Background

In the TSS-Roster proposal we explicitly pass a RosterHistory to the PlatformBuilder which contains the current and previous roster. While we are refactoring, it may be convenient to also initialize this class with AddressBook and return the current and previous AddressBook. Once we are done refactoring, the code related to AddressBook would be removed.

Constructors:

new RosterHistory(currentRoster, currentRound, previousRoster, previousRound) 
new RosterHistory(List<RoundRosterPair>, Map<RosterHash, Roster>)

Roster History API:

Roster       getCurrentRoster()
Roster       getPreviousRoster()
Roster       getRosterForRound(long round)

Acceptance Criteria

• Create unit tests that validate the behavior of the method calls for each constructor.

Dependencies

None