(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/server/lib/checksumFile.ts

@@ -0,0 +1,21 @@
+import {createHash} from 'crypto';
+import * as fs from 'fs';
+
+/**
+ * Computes hash of the file at the given path, using 'sha1' by default, or any algorithm
+ * supported by crypto.createHash().
+ */
+export async function checksumFile(filePath: string, algorithm: string = 'sha1'): Promise<string> {
+  const shaSum = createHash(algorithm);
+  const stream = fs.createReadStream(filePath);
+  try {
+    stream.on('data', (data) => shaSum.update(data));
+    await new Promise<void>((resolve, reject) => {
+      stream.on('end', resolve);
+      stream.on('error', reject);
+    });
+    return shaSum.digest('hex');
+  } finally {
+    stream.removeAllListeners();      // Isn't strictly necessary.
+  }
+}