* Problem: `ValidatorElection` and `MigrationElection` need to inherit from a common `Election` class.
Solution: Factored the common logic out of `ValidatorElection` and moved it to `Election` parent class.
* Problem: No need to store different types of elections in their own tables
Solution: Remove `DB_TABLE` property from `Election` class.
Solution: Created the `elections` table with secondary_index `election_id`.
* Problem: `UpsertValidatorVote` can be generalized to just be `Vote`
Solution: Renamed, refactored and moved the `Vote` class to tie in with the more general `Election` base class.
* Problem: `election_id` is not unique if two elections have the same properties.
Solution: Added a random `uuid4` seed to enforce uniqueness.
Solution: Change the docs to recommend installing a specific Tendermint version.
Also update our release process so we change that version number before a release, if necessary.
* Problem: we have unused and outdated fixtures
Solution: clean up fixtures and tests accordingly
* Problem: there are still unused fixtures
Solution: remove Merlin keys
* Problem: There are unnecessary markers
Solution: remove the tendermint marker for tests
* Problem: The commit mode behaves incorrectly.
Solution: Parse the Tendermint response properly. The functionality was disabled in https://github.com/bigchaindb/bigchaindb/pull/2235/files#diff-c6511560546a7dc577e7e647b5bfdaceL68 and was not fixed since then.
* Add a test case for the sync mode.
* Do not strictly expect deliver_tx in the response.
* Fix post_mock in web/test_transactions.py.
* Check for the error field first.
Solution: Document when the transactions posted in the async and sync modes can be lost. Describe how they are stored. Mention that BigchainDB does not store or expose rejected transactions.
The information on that page was either wrong or redundant, so I either deleted it or linked to where to find the same information in the docs about contributing to BigchainDB.
* Problem: No good way to check for val set absence.
Solution: Make get_validator_set/get_validators return None/[] when there are no validators yet.
* Problem: Incompatible ABCI chain upgrades.
Solution: Record known chains and sync through InitChain. Triggering the migration and adjusting other ABCI endpoints will follow.
Solution: Election ID has to be unique but not every validator set record has it. MongoDB does not support partial indexes, does not even allow for multiple Nones. This is a temporary fix since we are introducing an `election` collection to store election IDs in #2498.
* Problem: cyrptoconditions dependency updated because of vulnerability CVE-2018-10903
* update cc to ~=0.7.2
* Fix test using b58encode
* Fixing some more tests failing because of base58 update
