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

crypto/merkle/proof_key_path_test.go

@@ -1,6 +1,8 @@
 package merkle
 
 import (
+	// it is ok to use math/rand here: we do not need a cryptographically secure random
+	// number generator here and we can run the tests a bit faster
 	"math/rand"
 	"testing"
 
@@ -24,7 +26,7 @@ func TestKeyPath(t *testing.T) {
 					keys[i][j] = alphanum[rand.Intn(len(alphanum))]
 				}
 			case KeyEncodingHex:
-				rand.Read(keys[i])
+				rand.Read(keys[i]) //nolint: gosec
 			default:
 				panic("Unexpected encoding")
 			}