r17237: - keep pointer to the different sockets

r17237: - keep pointer to the different sockets
- we need this to later:
  - to disallow a StartTLS when TLS is already in use
  - to place the TLS socket between the raw and sasl socket
    when we had a sasl bind before the StartTLS
  - and rfc4513 says that the server may allow to remove the TLS from
    the tcp connection again and reuse raw tcp
  - and also a 2nd sasl bind should replace the old sasl socket

metze
(This used to be commit 10cb9c07ac60b03472f2b0b09c4581cc715002ba)