by --block-interval-seconds chain_plugin command line option. #733

- Rename config::ProducerCount -> config::BlocksPerRound
- Add new config::VotedProducersPerRound = 20
- Split up staked_balance_objects.hpp moving ProducerVotesObject to
producer_objects.hpp

Add support for producers specifying their votes on chain configuration
values to CreateProducer and UpdateProducer.

Add more tests around chain configuration, including a failing test case
verifying that the blockchain itself handles configuration updates
correctly. Presently this test fails because the chain does not yet
attempt to keep track of rounds, thus it cannot update the configuration
at the beginning of the round.

The protocol folder heralds back to BTS, when we drew a distinction
between "protocol objects" and "implementation objects." This
distinction was never really realized, and definitely isn't being
maintained now, so do away with the vestiges. It's nothing but a
potential point of confusion now.

Replace the C-style typless macros in config.hpp with C++-style const
definitions.

Everything related to the maintenance interval is now gone, including
(and especially, really) the skipped blocks during the maintenance
period.