HTTP2: Add a preference to enable/disable HTTP2 (weak) heuristics

By default, the heuristic is disabled

Change-Id: I26ef23e8b153576a4fabd2e3324e830756e64bb7
Ping-Bug:10335
Ping-Bug:10310
Reviewed-on: https://code.wireshark.org/review/3350
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>