Nest: Use bitmaps to keep track of exported routes

Use a hierarchical bitmap in a routing table to assign ids to routes, and
then use bitmaps (indexed by route id) in channels to keep track whether
routes were exported. This avoids unreliable and inefficient re-evaluation
of filters for old routes in order to determine whether they were exported.

lib/bitmap.c

@@ -26,6 +26,13 @@ bmap_init(struct bmap *b, pool *p, uint size)
   b->data = mb_allocz(p, b->size);
 }
 
+void
+bmap_reset(struct bmap *b, uint size)
+{
+  b->size = BIRD_ALIGN(size, 4);
+  memset(b->data, 0, b->size);
+}
+
 void
 bmap_grow(struct bmap *b, uint need)
 {