Don't use a separate LinkService for DAGService.GetLinks()

Instead make LinkService a part of DAGService.  The LinkService is now
simply an interface that DAGService implements.  Also provide a
GetOfflineLinkService() method that the GC uses to get an offline
instance.

License: MIT
Signed-off-by: Kevin Atkinson <k@kevina.org>

bitswap.go

@@ -422,3 +422,7 @@ func (bs *Bitswap) GetWantlist() []key.Key {
 	}
 	return out
 }
+
+func (bs *Bitswap) IsOnline() bool {
+	return true
+}