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

les/api_test.go

@@ -326,7 +326,8 @@ func testRequest(ctx context.Context, t *testing.T, client *rpc.Client) bool {
 	var res string
 	var addr common.Address
 	rand.Read(addr[:])
-	c, _ := context.WithTimeout(ctx, time.Second*12)
+	c, cancel := context.WithTimeout(ctx, time.Second*12)
+	defer cancel()
 	err := client.CallContext(c, &res, "eth_getBalance", addr, "latest")
 	if err != nil {
 		t.Log("request error:", err)