restructure: move bin/ and tests/ to src/

Also add symlinks from the old `bin/` and `tests/` locations to avoid
breaking scripts and other tools.

Motivations:

  * Scripts and tests no longer have to do dubious things like:

        require('ep_etherpad-lite/node_modules/foo')

    to access packages installed as dependencies in
    `src/package.json`.

  * Plugins can access the backend test helper library in a non-hacky
    way:

        require('ep_etherpad-lite/tests/backend/common')

  * We can delete the top-level `package.json` without breaking our
    ability to lint the files in `bin/` and `tests/`.

    Deleting the top-level `package.json` has downsides: It will cause
    `npm` to print warnings whenever plugins are installed, npm will
    no longer be able to enforce a plugin's peer dependency on
    ep_etherpad-lite, and npm will keep deleting the
    `node_modules/ep_etherpad-lite` symlink that points to `../src`.

    But there are significant upsides to deleting the top-level
    `package.json`: It will drastically speed up plugin installation
    because `npm` doesn't have to recursively walk the dependencies in
    `src/package.json`. Also, deleting the top-level `package.json`
    avoids npm's horrible dependency hoisting behavior (where it moves
    stuff from `src/node_modules/` to the top-level `node_modules/`
    directory). Dependency hoisting causes numerous mysterious
    problems such as silent failures in `npm outdated` and `npm
    update`. Dependency hoisting also breaks plugins that do:

        require('ep_etherpad-lite/node_modules/foo')

src/tests/backend/specs/promises.js

@@ -0,0 +1,85 @@
+const assert = require('assert').strict;
+const promises = require('../../../node/utils/promises');
+
+describe(__filename, function () {
+  describe('promises.timesLimit', function () {
+    let wantIndex = 0;
+    const testPromises = [];
+    const makePromise = (index) => {
+      // Make sure index increases by one each time.
+      assert.equal(index, wantIndex++);
+      // Save the resolve callback (so the test can trigger resolution)
+      // and the promise itself (to wait for resolve to take effect).
+      const p = {};
+      const promise = new Promise((resolve) => {
+        p.resolve = resolve;
+      });
+      p.promise = promise;
+      testPromises.push(p);
+      return p.promise;
+    };
+
+    const total = 11;
+    const concurrency = 7;
+    const timesLimitPromise = promises.timesLimit(total, concurrency, makePromise);
+
+    it('honors concurrency', async function () {
+      assert.equal(wantIndex, concurrency);
+    });
+
+    it('creates another when one completes', async function () {
+      const {promise, resolve} = testPromises.shift();
+      resolve();
+      await promise;
+      assert.equal(wantIndex, concurrency + 1);
+    });
+
+    it('creates the expected total number of promises', async function () {
+      while (testPromises.length > 0) {
+        // Resolve them in random order to ensure that the resolution order doesn't matter.
+        const i = Math.floor(Math.random() * Math.floor(testPromises.length));
+        const {promise, resolve} = testPromises.splice(i, 1)[0];
+        resolve();
+        await promise;
+      }
+      assert.equal(wantIndex, total);
+    });
+
+    it('resolves', async function () {
+      await timesLimitPromise;
+    });
+
+    it('does not create too many promises if total < concurrency', async function () {
+      wantIndex = 0;
+      assert.equal(testPromises.length, 0);
+      const total = 7;
+      const concurrency = 11;
+      const timesLimitPromise = promises.timesLimit(total, concurrency, makePromise);
+      while (testPromises.length > 0) {
+        const {promise, resolve} = testPromises.pop();
+        resolve();
+        await promise;
+      }
+      await timesLimitPromise;
+      assert.equal(wantIndex, total);
+    });
+
+    it('accepts total === 0, concurrency > 0', async function () {
+      wantIndex = 0;
+      assert.equal(testPromises.length, 0);
+      await promises.timesLimit(0, concurrency, makePromise);
+      assert.equal(wantIndex, 0);
+    });
+
+    it('accepts total === 0, concurrency === 0', async function () {
+      wantIndex = 0;
+      assert.equal(testPromises.length, 0);
+      await promises.timesLimit(0, 0, makePromise);
+      assert.equal(wantIndex, 0);
+    });
+
+    it('rejects total > 0, concurrency === 0', async function () {
+      await assert.rejects(promises.timesLimit(total, 0, makePromise), RangeError);
+    });
+  });
+});