Rewrite dispatch() to benefit from async/await (less error prone)

[punkeel]

• Make dispatch() an async function, and await every async function call.
  By removing Promise.xxx, we reduce the chance that an async function
  is called without (a)waiting for it to end, which in turn prevents
  uncaught exceptions.

• Simplify code to let errors propagate upwards, without catching them.

• Use 'dispatch.bind' in the recursion to remove a function from the
  stack trace. This binding trick was done in Koa:
  koajs/compose@13768ff

  To support this, we move the params-reset to the end of dispatch().

No tests were harmed in the making of this change.

I am opening this PR because I suspect some exceptions aren't caught by dispatch() and instead cause uncaught exceptions. I do not have a simple example of this happening, but I suspect it happens here:

8track/src/Router.ts

Line 226 in cb4a121

route.handler(ctx, async () => {

route.handler is an async function, 8track does not await it, and it leaks. The try-catch doesn't see it either, as it's happening in the async world.... or maybe if (p) return p.then(() => ctx.response), we don't attach a .then(fn, fn) nor .catch(fn)...

[jrf0110]

Thanks for this! I've always disliked how inscrutable this function was. I'm going to play around with your change and will likely publish to a release candidate flag for testing.