Proper threshold support

[fjarri]

Fixes #11

Done:

• introduced quorum concept in the API, using it for normal rounds

TODO:

• a method of IdSet that would tell us if quorum can never be reached given the already banned nodes
• don't expect messages from a node if it was already banned in the previous round
• support threshold in the echo round, and figure out what CommunicationInfo should be like for that
• generalize IdSet to be a trait, allowing users to specify their own quorum conditions?
• depending on how the echo round is handled, may be able to close Decouple protocol and session levels #25?

[coveralls]

Pull Request Test Coverage Report for Build 13447280574

Details

• 196 of 268 (73.13%) changed or added relevant lines in 10 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.8%) to 72.643%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/combinators/misbehave.rs	23	25	92.0%
manul/src/protocol/round.rs	32	34	94.12%
manul/src/session/session.rs	44	53	83.02%
manul/src/combinators/chain.rs	33	45	73.33%
manul/src/protocol/boxed_round.rs	24	38	63.16%
manul/src/session/evidence.rs	8	23	34.78%
manul/src/session/echo.rs	7	25	28.0%

Files with Coverage Reduction	New Missed Lines	%
manul/src/session/evidence.rs	1	53.75%
manul/src/protocol/errors.rs	3	16.09%
manul/src/session/session.rs	6	77.52%

Totals
Change from base Build 13401865846:	-0.8%
Covered Lines:	2196
Relevant Lines:	3023

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💛 - Coveralls