types: change Commit to consist of just signatures (#4146)

* types: change `Commit` to consist of just signatures

These are final changes towards removing votes from commit and leaving
only signatures (see ADR-25)

Fixes #1648

* bring back TestCommitToVoteSetWithVotesForAnotherBlockOrNilBlock

+ add absent flag to Vote to indicate that it's for another block

* encode nil votes as CommitSig with BlockIDFlagAbsent

+ make Commit#Precommits array of non-pointers
because precommit will never be nil

* add NewCommitSigAbsent and Absent() funcs

* uncomment validation in CommitSig#ValidateBasic

* add comments to ValidatorSet funcs

* add a changelog entry

* break instead of continue

continue does not make sense in these cases

* types: rename Commit#Precommits to Signatures

* swagger: fix /commit response

* swagger: change block_id_flag type

* fix merge conflicts

store/store_test.go

@@ -29,8 +29,13 @@ type cleanupFunc func()
 
 // make a Commit with a single vote containing just the height and a timestamp
 func makeTestCommit(height int64, timestamp time.Time) *types.Commit {
-	commitSigs := []*types.CommitSig{{Height: height, Timestamp: timestamp}}
-	return types.NewCommit(types.BlockID{}, commitSigs)
+	commitSigs := []types.CommitSig{{
+		BlockIDFlag:      types.BlockIDFlagCommit,
+		ValidatorAddress: []byte("ValidatorAddress"),
+		Timestamp:        timestamp,
+		Signature:        []byte("Signature"),
+	}}
+	return types.NewCommit(height, 0, types.BlockID{}, commitSigs)
 }
 
 func makeTxs(height int64) (txs []types.Tx) {