fix blocking of tileserver while serving index

[candux]

we noticed sporadic long blocks of the entier tileserver on version 3.1.1
They always happend when somebody accessed the frontPage.
Our guess is, that clone() blocks the object and takes very long to finish.

We replaced the clone with ES6 magic from https://stackoverflow.com/a/42988763
and the problem is gone.

Also the time to generate the index goes from ~3s to ~0.2s
This should primarily affect user with many large styles

[mnutt]

Object.assign() does a shallow copy, meaning only the top-level styles object is duplicated, while the clone module does a deep clone. Looking at the code following it, I think it's fine to leave the top-level alone and Object.assign the individual styles:

serveTemplate('/$', 'index', req => {
    for (const id of Object.keys(serving.styles || {})) {
      const style = Object.assign({}, styles[id]);
      style.name = (serving.styles[id] || serving.rendered[id] || {}).name;
      style.serving_data = serving.styles[id];
      style.serving_rendered = serving.rendered[id];
      ...

Then the same can be done with data.

[acalcutt]

This should be fixed by #1009 . In that PR I have added a similar fix to index which does a more shallow array copy in https://github.com/WifiDB/tileserver-gl/blob/fa3dac46ca69ede31635867c4612dbf1a5b5decc/src/server.js#L450-L538

I am closing this PR now since it is no longer needed.