dtls: use SslSession

Use the struct SslSession instead of passing the tls version and cipher
to each function.

Change-Id: I95ad8cb5857794608f0f8db5c2dfd4b16e6578d5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://code.wireshark.org/review/1820
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit 00251edd7d6bb2720507fa30fcde4d4e2f6c4842)
Reviewed-on: https://code.wireshark.org/review/2813
Reviewed-by: Anders Broman <a.broman58@gmail.com>