Move base64 to utils.serialization

I don't want to re-write `b64encode/b64decode` functions, but when I import them from `celery.security.serialization` I also import OpenSSL, which is very slow and cause almost 1 million calls (from cProfile):
```
996542 function calls (972825 primitive calls) in 0.971 seconds
```
With my patch, importing these functions from `celery.utils.serialization` results in almost 8x times speed up:
```
127645 function calls (122927 primitive calls) in 0.179 seconds
```