make gosec linter pass (#3294)

* not related to linter: remove obsolete constants:
 - `Insecure` and `Secure` and type `Security` are not used anywhere

* not related to linter: update example

 - NewInsecure was deleted; change example to NewRemoteDB

* address: Binds to all network interfaces (gosec):

 - bind to localhost instead of 0.0.0.0
 - regenerate test key and cert for this purpose (was valid for ::) and
 otherwise we would see:
 transport: authentication handshake failed: x509: certificate is
 valid for ::, not 127.0.0.1\"

(used https://github.com/google/keytransparency/blob/master/scripts/gen_server_keys.sh
to regenerate certs)

* use sha256 in tests instead of md5; time difference is negligible

* nolint usage of math/rand in test and add comment on its import

 - crypto/rand is slower and we do not need sth more secure in tests

* enable linter in circle-ci

* another nolint math/rand in test

* replace another occurrence of md5

* consistent comment about importing math/rand

mempool/mempool_test.go

@@ -1,8 +1,8 @@
 package mempool
 
 import (
-	"crypto/md5"
 	"crypto/rand"
+	"crypto/sha256"
 	"encoding/binary"
 	"fmt"
 	"io/ioutil"
@@ -451,7 +451,7 @@ func TestMempoolMaxMsgSize(t *testing.T) {
 }
 
 func checksumIt(data []byte) string {
-	h := md5.New()
+	h := sha256.New()
 	h.Write(data)
 	return fmt.Sprintf("%x", h.Sum(nil))
 }