fib: FIB Entry tracking

Instead of all clients directly RR sourcing the entry they are tracking,
use a deidcated 'tracker' object. This tracker object is a entry
delegate and a child of the entry. The clients are then children of the
tracker.
The benefit of this aproach is that each time a new client tracks the
entry it doesn't RR source it. When an entry is sourced all its children
are updated. Thus, new clients tracking an entry is O(n^2). With the
tracker as indirection, the entry is sourced only once.

Type: feature

Change-Id: I5b80bdda6c02057152e5f721e580e786cd840a3b
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/CMakeLists.txt b/src/vnet/CMakeLists.txt
index 7af1703..68e806e 100644
--- a/src/vnet/CMakeLists.txt
+++ b/src/vnet/CMakeLists.txt
@@ -1288,6 +1288,7 @@
   fib/fib_entry_src_lisp.c
   fib/fib_entry_cover.c
   fib/fib_entry_delegate.c
+  fib/fib_entry_track.c
   fib/fib_path_list.c
   fib/fib_path.c
   fib/fib_path_ext.c