imap: TLS upgrade fix

imap: TLS upgrade fix

There were two places in the code that tried to connect the SSL filter,
e.g. do the TLS handshake, but only one changed imap state to CAPA
afterwards.

Depending on timing, the wrong path was taken and the connection was
hanging, waiting for a server reply to a command not sent.

Do the upgrade to tls in one place and update connection filter and
smtps protocol handler at the same time. Always transition to CAPA on
success.

Closes #16213