* Fix bug in inCurrentEpoch and improve accumulator related tests
- Fix negative wraparound / underflow in inCurrentEpoch
- Add tests in accumulator tests to verify the above
- Add header offer tests with accumulator that does and doesn't
contain historical epochs
- Additional clean-up of history tests
- enable canonicalVerify in the tests
- Move the accumulator definitions to a history accumulator file
- Add accumulator build helper calls + temporary database
- Add a header gossip content key encoding test
- Refactor & some cleanup