Rework upsert-validator show status (#2496)

* 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

tests/web/test_validators.py

@@ -14,7 +14,7 @@ def test_get_validators_endpoint(b, client):
                       'pub_key': {'data': '4E2685D9016126864733225BE00F005515200727FBAB1312FC78C8B76831255A',
                                   'type': 'ed25519'},
                       'voting_power': 10}]
-    b.store_validator_set(23, validator_set)
+    b.store_validator_set(23, validator_set, 'election_id')
 
     res = client.get(VALIDATORS_ENDPOINT)
     assert is_validator(res.json[0])