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

node/node_test.go

@@ -279,7 +279,7 @@ func TestCreateProposalBlock(t *testing.T) {
 		evidencePool,
 	)
 
-	commit := types.NewCommit(types.BlockID{}, nil)
+	commit := types.NewCommit(height-1, 0, types.BlockID{}, nil)
 	block, _ := blockExec.CreateProposalBlock(
 		height,
 		state, commit,