(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/ActionRouter.ts

@@ -0,0 +1,49 @@
+import { Rpc } from "grain-rpc";
+
+/**
+ * ActionRouter allows to choose what actions to send over rpc. Action are posted as message `{type:
+ * "docAction", action }` over rpc.
+ */
+export class ActionRouter {
+
+  private _subscribedTables: Set<string> = new Set();
+
+  constructor(private _rpc: Rpc) {}
+
+  /**
+   * Subscribe to send all actions related to a table. Keeps sending actions if table is renamed.
+   */
+  public subscribeTable(tableId: string): Promise<void> {
+    this._subscribedTables.add(tableId);
+    return Promise.resolve();
+  }
+
+  /**
+   * Stop sending all message related to a table.
+   */
+  public unsubscribeTable(tableId: string): Promise<void> {
+    this._subscribedTables.delete(tableId);
+    return Promise.resolve();
+  }
+
+  /**
+   * Process a action updates subscription set in case of table rename and table remove, and post
+   * action if it matches a subscriptions.
+   */
+  public process(action: any[]): Promise<void> {
+    const tableId = action[1];
+    if (!this._subscribedTables.has(tableId)) {
+      return Promise.resolve();
+    }
+    switch (action[0]) {
+      case "RemoveTable":
+        this._subscribedTables.delete(tableId);
+        break;
+      case "RenameTable":
+        this._subscribedTables.delete(tableId);
+        this._subscribedTables.add(action[2]);
+        break;
+    }
+    return this._rpc.postMessage({type: "docAction", action});
+  }
+}