ssl: add SslSession structure

This structure is used to store information about a SSL session which
is not only needed for decrypting the session, but also to show nice
dissection information.

In an other patch I will add some more members to the struct because
the old way of passing them to the function does not scale.

Change-Id: I88e7f2896e0364a41d4538752dad291de83bfbca
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://code.wireshark.org/review/1819
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit a7a4aa9a743f68f9864011d3b56bb5739c2cda15)
Reviewed-on: https://code.wireshark.org/review/2812
Reviewed-by: Anders Broman <a.broman58@gmail.com>