[orchagent] Add separate next hop table and orch (sonic-net#1702)

Added a new next hop group table to APP_DB, and orchagent support
to use this as an alternative to including next hop information in
the route table. Added a new "nghOrch" that subscribes to next hop group
table events to program next hop groups into SAI layer.

Improves performance and occupancy usage when using the new table

Extended SWSS unit tests to cover new code, and ran existing tests

doc/swss-schema.md

@@ -159,8 +159,35 @@ and reflects the LAG ports into the redis under: `LAG_TABLE:<team0>:port`
     ;Status: Mandatory
     key           = ROUTE_TABLE:prefix
     nexthop       = *prefix, ;IP addresses separated “,” (empty indicates no gateway)
-    intf          = ifindex? PORT_TABLE.key  ; zero or more separated by “,” (zero indicates no interface)
+    ifname        = ifindex? PORT_TABLE.key  ; zero or more separated by “,” (zero indicates no interface)
+    mpls_nh       = STRING                   ; Comma-separated list of MPLS NH info.
     blackhole     = BIT ; Set to 1 if this route is a blackhole (or null0)
+    weight        = weight_list              ; List of weights.
+    nexthop_group = string ; index within the NEXTHOP_GROUP_TABLE, used instead of nexthop and intf fields
+
+---------------------------------------------
+
+###### LABEL_ROUTE_TABLE
+    ; Defines schema for MPLS label route table attributes
+    key           = LABEL_ROUTE_TABLE:mpls_label ; MPLS label
+    ; field       = value
+    nexthop       = STRING                   ; Comma-separated list of nexthops.
+    ifname        = STRING                   ; Comma-separated list of interfaces.
+    mpls_nh       = STRING                   ; Comma-separated list of MPLS NH info.
+    mpls_pop      = STRING                   ; Number of ingress MPLS labels to POP
+    weight        = STRING                   ; Comma-separated list of weights.
+    blackhole     = BIT ; Set to 1 if this route is a blackhole (or null0)
+    nexthop_group = string ; index within the NEXTHOP_GROUP_TABLE, used instead of nexthop and intf fields
+
+---------------------------------------------
+### NEXTHOP_GROUP_TABLE
+    ;Stores a list of groups of one or more next hops
+    ;Status: Mandatory
+    key           = NEXTHOP_GROUP_TABLE:string ; arbitrary index for the next hop group
+    nexthop       = *prefix, ;IP addresses separated “,” (empty indicates no gateway)
+    ifname        = ifindex? PORT_TABLE.key  ; zero or more separated by “,” (zero indicates no interface)
+    mpls_nh       = STRING                   ; Comma-separated list of MPLS NH info.
+    weight        = weight_list              ; List of weights.
 
 ---------------------------------------------
 ### NEIGH_TABLE

orchagent/Makefile.am

@@ -39,6 +39,7 @@ orchagent_SOURCES = \
             orchdaemon.cpp \
             orch.cpp \
             notifications.cpp \
+            nhgorch.cpp \ 
             routeorch.cpp \
             mplsrouteorch.cpp \
             neighorch.cpp \