cstp: Always restart DTLS after a CSTP reconnection

cstp_reconnect() can get called on its own, e.g. due to a DPD timeout.
Under some circumstances this can cause the DTLS parameters to be
renegotiated.  For instance, start_cstp_connection() sometimes sends a
new master DTLS secret to the gateway.  And Cisco ASAs seem to like
generating new DTLS-Session-ID strings on each reconnect.

If DTLS is not restarted, the client and server will get out of sync and
will not be able to pass data traffic.  The situation is exacerbated by
certain gateway settings, like setting DPD=0 and disabling DTLS rekey,
which may cause the connection to remain in a "living dead" state for
extended periods of time.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>