build: use golangci-lint (#20295)

* build: use golangci-lint

This changes build/ci.go to download and run golangci-lint instead
of gometalinter.

* core/state: fix unnecessary conversion

* p2p/simulations: fix lock copying (found by go vet)

* signer/core: fix unnecessary conversions

* crypto/ecies: remove unused function cmpPublic

* core/rawdb: remove unused function print

* core/state: remove unused function xTestFuzzCutter

* core/vm: disable TestWriteExpectedValues in a different way

* core/forkid: remove unused function checksum

* les: remove unused type proofsData

* cmd/utils: remove unused functions prefixedNames, prefixFor

* crypto/bn256: run goimports

* p2p/nat: fix goimports lint issue

* cmd/clef: avoid using unkeyed struct fields

* les: cancel context in testRequest

* rlp: delete unreachable code

* core: gofmt

* internal/build: simplify DownloadFile for Go 1.11 compatibility

* build: remove go test --short flag

* .travis.yml: disable build cache

* whisper/whisperv6: fix ineffectual assignment in TestWhisperIdentityManagement

* .golangci.yml: enable goconst and ineffassign linters

* build: print message when there are no lint issues

* internal/build: refactor download a bit

core/vm/instructions_test.go

@@ -232,7 +232,9 @@ func getResult(args []*twoOperandParams, opFn executionFunc) []TwoOperandTestcas
 
 // utility function to fill the json-file with testcases
 // Enable this test to generate the 'testcases_xx.json' files
-func xTestWriteExpectedValues(t *testing.T) {
+func TestWriteExpectedValues(t *testing.T) {
+	t.Skip("Enable this test to create json test cases.")
+
 	for name, method := range twoOpMethods {
 		data, err := json.Marshal(getResult(commonParams, method))
 		if err != nil {
@@ -243,7 +245,6 @@ func xTestWriteExpectedValues(t *testing.T) {
 			t.Fatal(err)
 		}
 	}
-	t.Fatal("This test should not be activated")
 }
 
 // TestJsonTestcases runs through all the testcases defined as json-files