Candidate Agreement + Consensus (#29)

* candidate statement importing

* import votes on validity

* import availability votes

* candidate receipt type

* make table mod public

* test context for table

* add harness for tests

* some tests for misbehavior

* produce proposal from table

* count candidate issuance as implicit vote

* keep track of messages known by validators

* fix primitives compilation

* simple BFT agreement

* kill unused macro_use annotation

* tests for BFT agreement

* test for not concluding on different prepares

* return summary upon statement import

* accept bft agreement on proposal not locally submitted

* check justification set for BFT

* BFT rewrite: vote accumulator with tests

* squash some warnings

* a few more tests for the accumulator

* add sender to table's signed statement

* implement honest node strategy for BFT

* inex -> index

* import and broadcast lock proofs

* poll repeatedly when state changes

* ...