dtls: use own buffer to store CCS message

Do not use the sendbuf for the Change Cipher Spec message any more.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: remove sendbuf

The sendbuf is not used by anycode any more.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>