Reset cStack's C on release

Previously, we would keep the URLParams / Env associated with a cStack
around until the next request flushed them. However, this might cause
either of these maps to stick around for much longer than they ought to,
potentially keeping references to many, many objects.

Instead, clear out the saved context on every release.

web/middleware.go

@@ -130,6 +130,7 @@ func (m *mStack) alloc() *cStack {
 }
 
 func (m *mStack) release(cs *cStack) {
+	cs.C = C{}
 	if cs.pool != m.pool {
 		return
 	}