Support delivering initial cache.watch results immediately, if reques…

…ted.

I needed this functionality for some tests of cache.modify, but I think I
can expand it into a version of cache.watch that returns an async iterator
if you do not supply a callback.

src/cache/core/types/Cache.ts

@@ -21,6 +21,7 @@ export namespace Cache {
   }
 
   export interface WatchOptions extends ReadOptions {
+    immediate?: boolean;
     callback: WatchCallback;
   }
 

src/cache/inmemory/inMemoryCache.ts

@@ -165,7 +165,9 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
 
   public watch(watch: Cache.WatchOptions): () => void {
     this.watches.add(watch);
-
+    if (watch.immediate) {
+      this.maybeBroadcastWatch(watch);
+    }
     return () => {
       this.watches.delete(watch);
     };