Switch usage of math/rand to cmn's rand (#1980)

This commit switches all usage of math/rand to cmn's rand. The only
exceptions are within the random file itself, the tools package, and the
crypto package. In tools you don't want it to lock between the go-routines.
The crypto package doesn't use it so the crypto package have no other
dependencies within tendermint/tendermint for easier portability.

Crypto/rand usage is unadjusted.

Closes #1343

libs/common/os_test.go

@@ -3,17 +3,14 @@ package common
 import (
 	"bytes"
 	"io/ioutil"
-	"math/rand"
 	"os"
 	"testing"
-	"time"
 )
 
 func TestWriteFileAtomic(t *testing.T) {
 	var (
-		seed             = rand.New(rand.NewSource(time.Now().UnixNano()))
-		data             = []byte(RandStr(seed.Intn(2048)))
-		old              = RandBytes(seed.Intn(2048))
+		data             = []byte(RandStr(RandIntn(2048)))
+		old              = RandBytes(RandIntn(2048))
 		perm os.FileMode = 0600
 	)