* 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.
* Problem: we have unused and outdated fixtures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: 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: We need to store the `election_id` as part of the `validator_update` so we can efficiently check which election was resposible for the change
Solution: Added the parameter to `store_validator_set` and aligned the tests
* Problem: Logic for `upsert-validator show` is convoluted
Solution: Rewrote the function to be much simpler
* Problem: Need a uniqueness constraint for election_id wrt validator changes
Solution: Added a new key to the db schema
* Problem: Source files contain no license info
Solution: Add comments with SPDX license info to source files
* Python 3 files don't need # -*- coding: utf-8 -*-
* Problem: Validator set not tracked by BigchainDB
Solution: BigchainDB depends on tendermint's RPC API to get the validator set
which is not avaiable during replay so the validators set should be tracked
inside BigchainDB
* Problem: Unclear code and documentation
Solution: Fix decode_validator and docs strings
* Problem: Doc strings missing
Solution: Add doc string for store_validato_set
