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

types/block_test.go

@@ -1,6 +1,8 @@
 package types
 
 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
 	"crypto/rand"
 	"math"
 	"os"
@@ -162,8 +164,8 @@ func TestBlockString(t *testing.T) {
 func makeBlockIDRandom() BlockID {
 	blockHash := make([]byte, tmhash.Size)
 	partSetHash := make([]byte, tmhash.Size)
-	rand.Read(blockHash)
-	rand.Read(partSetHash)
+	rand.Read(blockHash)   //nolint: gosec
+	rand.Read(partSetHash) //nolint: gosec
 	blockPartsHeader := PartSetHeader{123, partSetHash}
 	return BlockID{blockHash, blockPartsHeader}
 }