418892 - SSL session caching so unreliable it effectively does not
work.

Fixed by making sure that we completely decrypt read bytes.

Before the fix, it was possible that we returned after the decryption
of one TLS frame, while another was still present in the
_encryptedBuffer.
This lead to non-clean closes of the connection, which hampered the
capability of session reuse by clients.

Now we decrypt in a loop and only return if there is nothing more
that we can decrypt.