Added basic conversation support

Loosely based on a work done by Pascal Quantin <pascal.quantin@gmail.com> (see
svn rev. 52306).

This was backported from master. See this link for the original patch:

https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=9211

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>