NAT44: endpoint dependent mode (VPP-1273)

NAT44: endpoint dependent mode (VPP-1273)

To enable NAT plugin endpoint dependent mode add following to statrup config:
nat { endpoint-dependent }

Enable endpoint dependent filtering and mapping for all sessions.
Move some existing functionality such as service load balancing, twice nat,
out2in-only static mappings and unknown protocol dynamic translations, which
use endpoint dependent lookup hash tables before. Basically split to vanilla
NAT44 and extra features NAT44.

Change-Id: I3925eb5ddcc8f1ec4cf6af4e2a618a7ec7aa9735
Signed-off-by: Matus Fabian <matfabia@cisco.com>