enable unconvert, goconst and nakedret linters (#3973)

This should've been a part of
https://github.com/tendermint/tendermint/pull/3960, but I forgot about
it while reviewing.

A good programmer is someone who always looks both ways before crossing
a one-way street. - Doug Linder

.golangci.yml

@@ -9,10 +9,7 @@ linters:
     - maligned
     - errcheck
     - interfacer
-    - unconvert
-    - goconst
     - unparam
-    - nakedret
     - lll
     - gochecknoglobals
     - gochecknoinits
@@ -29,9 +26,6 @@ linters:
 #     suggest-new: true
 #   dupl:
 #     threshold: 100
-#   goconst:
-#     min-len: 2
-#     min-occurrences: 2
 #   depguard:
 #     list-type: blacklist
 #     packages:

types/vote_set.go

@@ -434,7 +434,7 @@ func (voteSet *VoteSet) StringIndented(indent string) string {
 	voteStrings := make([]string, len(voteSet.votes))
 	for i, vote := range voteSet.votes {
 		if vote == nil {
-			voteStrings[i] = "nil-Vote"
+			voteStrings[i] = nilVoteStr
 		} else {
 			voteStrings[i] = vote.String()
 		}
@@ -499,7 +499,7 @@ func (voteSet *VoteSet) voteStrings() []string {
 	voteStrings := make([]string, len(voteSet.votes))
 	for i, vote := range voteSet.votes {
 		if vote == nil {
-			voteStrings[i] = "nil-Vote"
+			voteStrings[i] = nilVoteStr
 		} else {
 			voteStrings[i] = vote.String()
 		}