indexer: GetEnvelope now includes VoterID in returned envelope

[altergui]

Until now, the returned envelope was missing this field,
causing the endpoint GET /votes/{voteId} to return a nil VoterID

[altergui]

i dug into this while working on #1028

vocdoni-node/test/api_test.go

Lines 184 to 185 in af85c05

	// TODO (painan): check why the voterID is not present on the reply
	//qt.Assert(t, v2.VoterID.String(), qt.Equals, voterKey.AddressString())

i'm not uncommenting that line on this PR (now it works) since i'm refactoring the whole thing on #1028