[vnetorch] ECMP for vnet tunnel routes with endpoint health monitor (sonic-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.

Author: shi-su

orchagent/orchdaemon.cpp

@@ -122,6 +122,8 @@ bool OrchDaemon::init()
     TableConnector stateDbFdb(m_stateDb, STATE_FDB_TABLE_NAME);
     TableConnector stateMclagDbFdb(m_stateDb, STATE_MCLAG_REMOTE_FDB_TABLE_NAME);
     gFdbOrch = new FdbOrch(m_applDb, app_fdb_tables, stateDbFdb, stateMclagDbFdb, 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,
@@ -309,9 +311,6 @@ bool OrchDaemon::init()
 
     gMacsecOrch = new MACsecOrch(m_applDb, m_stateDb, macsec_app_tables, gPortsOrch);
 
-    TableConnector stateDbBfdSessionTable(m_stateDb, STATE_BFD_SESSION_TABLE_NAME);
-    gBfdOrch = new BfdOrch(m_applDb, APP_BFD_SESSION_TABLE_NAME, stateDbBfdSessionTable);
-
     gNhgMapOrch = new NhgMapOrch(m_applDb, APP_FC_TO_NHG_INDEX_MAP_TABLE_NAME);
 
     /*