[vnetorch] ECMP for vnet tunnel routes with endpoint health monitor (s…

…onic-net#1955)

What I did
Add functions to create/remove next hop groups for vnet tunnel routes.
Count the reference count of next hop groups to create and remove as needed.
Share the counter of next hop groups with routeorch.
Adapt route endpoint according to the BFD state of endpoints.

Why I did it
To add support for overlay ECMP.

How I verified it
Verify ECMP groups are properly created and removed with the functions.

orchagent/orchdaemon.cpp

@@ -107,6 +107,8 @@ bool OrchDaemon::init()
     gPortsOrch = new PortsOrch(m_applDb, ports_tables);
     TableConnector stateDbFdb(m_stateDb, STATE_FDB_TABLE_NAME);
     gFdbOrch = new FdbOrch(m_applDb, app_fdb_tables, stateDbFdb, gPortsOrch);
+    TableConnector stateDbBfdSessionTable(m_stateDb, STATE_BFD_SESSION_TABLE_NAME);
+    gBfdOrch = new BfdOrch(m_applDb, APP_BFD_SESSION_TABLE_NAME, stateDbBfdSessionTable);
 
     vector<string> vnet_tables = {
             APP_VNET_RT_TABLE_NAME,
@@ -264,9 +266,6 @@ bool OrchDaemon::init()
     MuxStateOrch *mux_st_orch = new MuxStateOrch(m_stateDb, STATE_HW_MUX_CABLE_TABLE_NAME);
     gDirectory.set(mux_st_orch);
 
-    TableConnector stateDbBfdSessionTable(m_stateDb, STATE_BFD_SESSION_TABLE_NAME);
-    gBfdOrch = new BfdOrch(m_applDb, APP_BFD_SESSION_TABLE_NAME, stateDbBfdSessionTable);
-
     /*
      * The order of the orch list is important for state restore of warm start and
      * the queued processing in m_toSync map after gPortsOrch->allPortsReady() is set.