Prefer const wherever possible

Summary:
Non-final identifiers make code harder to understand.
This is particularly true for JavaScript where even the *type*
can change as a value gets reassigned later.

This enforces to use `const` whereever possible, but doesn't
"outlaw" `let`. Mixed destructuring is also still allowed.

Used `eslint --fix` to change all existing cases.

Reviewed By: jknoxville

Differential Revision: D16131329

fbshipit-source-id: 2eceaca7c603b71b36e005be5d135e1849f2518d

src/server.js

@@ -76,7 +76,7 @@ export default class Server extends EventEmitter {
   ): Promise<RSocketServer> {
     const server = this;
     return new Promise((resolve, reject) => {
-      let rsServer;
+      let rsServer; // eslint-disable-line prefer-const
       const serverFactory = onConnect => {
         const transportServer = sslConfig
           ? tls.createServer(sslConfig, socket => {