session: make session code compile with SESSION_DEBUG enable

session: make session code compile with SESSION_DEBUG enable

Session debug code does not compile anymore due to vlib_mains global
variable disappearing over time. Replace it with vlib_get_main_by_index
call.

Add a cmake variable and pass it from make command line to enable
session debug. Notice transport debug is required for session debug.

make rebuild VPP_EXTRA_CMAKE_ARGS=-DVPP_TCP_DEBUG_ALWAYS=ON VPP_EXTRA_CMAKE_ARGS+=-DVPP_SESSION_DEBUG=ON

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Ic2e887c6b10b77cbabd56934f4931fcfa04a6751