Add headless option to dump on disconnect

Summary:
This adds an optional exit strategy that reacts to the client disconnecting rather than a `SIGINT` which can be used for integration tests.

`MiddlewareAPI` is a subset of `Store` and required to work here.

Annoyingly, it's not quite clear to me why this does not work as part of an event loop cycle and requires a `setTimeout`. This doesn't have any negative effects and works in the same way that the SIGINT interruption works, but it's a bit of an eyesore.

Reviewed By: danielbuechele

Differential Revision: D14622111

fbshipit-source-id: e2caca056e478428d977565dc9bc09eefca4230c

src/plugins/layout/index.js

@@ -9,7 +9,7 @@ import type {
   ElementID,
   Element,
   ElementSearchResultSet,
-  Store,
+  MiddlewareAPI,
   PluginClient,
 } from 'flipper';
 
@@ -61,7 +61,7 @@ export default class Layout extends FlipperPlugin<State, void, PersistedState> {
   static exportPersistedState = (
     callClient: (string, ?Object) => Promise<Object>,
     persistedState: ?PersistedState,
-    store: ?Store,
+    store: ?MiddlewareAPI,
   ): Promise<?PersistedState> => {
     const defaultPromise = Promise.resolve(persistedState);
     if (!store) {