Natsyncd changes in sonic-swss submodule to support NAT feature.

 - Added natsyncd and warmboot related changes.

Link to NAT HLD : https://github.com/Azure/SONiC/blob/master/doc/nat/nat_design_spec.md

Depends on:
sonic-swss :
sonic-swss-common : sonic-net/sonic-swss-common#304
sonic-linux-kernel : sonic-net/sonic-linux-kernel#100
sonic-sairedis : sonic-net/sonic-sairedis#519

.gitignore

@@ -56,6 +56,7 @@ cfgmgr/nbrmgrd
 cfgmgr/vxlanmgrd
 neighsyncd/neighsyncd
 portsyncd/portsyncd
+natsyncd/natsyncd
 orchagent/orchagent
 orchagent/routeresync
 orchagent/orchagent_restart_check

Makefile.am

@@ -1,4 +1,4 @@
-SUBDIRS = fpmsyncd neighsyncd portsyncd orchagent swssconfig cfgmgr tests
+SUBDIRS = fpmsyncd neighsyncd portsyncd natsyncd orchagent swssconfig cfgmgr tests
 
 if HAVE_LIBTEAM
 SUBDIRS += teamsyncd

configure.ac

@@ -83,6 +83,7 @@ AC_CONFIG_FILES([
     orchagent/Makefile
     fpmsyncd/Makefile
     neighsyncd/Makefile
+    natsyncd/Makefile
     portsyncd/Makefile
     teamsyncd/Makefile
     swssconfig/Makefile

natsyncd/Makefile.am

@@ -0,0 +1,16 @@
+INCLUDES = -I $(top_srcdir) -I $(top_srcdir)/warmrestart
+
+bin_PROGRAMS = natsyncd
+
+if DEBUG
+DBGFLAGS = -ggdb -DDEBUG
+else
+DBGFLAGS = -g
+endif
+
+natsyncd_SOURCES = natsyncd.cpp natsync.cpp $(top_srcdir)/warmrestart/warmRestartAssist.cpp
+
+natsyncd_CFLAGS = $(DBGFLAGS) $(AM_CFLAGS) $(CFLAGS_COMMON)
+natsyncd_CPPFLAGS = $(DBGFLAGS) $(AM_CFLAGS) $(CFLAGS_COMMON)
+natsyncd_LDADD = -lnl-3 -lnl-route-3 -lnl-nf-3 -lswsscommon
+