Improve performance by using less setImmediate calls

[SimonVanneste]

setImmediate has to be polyfilled in just about every browser (caniuse) by using the post message trick (see e.g. https://github.com/YuzuJS/setImmediate/blob/master/setImmediate.js)
This is however a huge slowdown:

(This example of a MergeIterator looks so sparse because of setImmediate calls)

• Minimise usage of BufferedIterator
  A BufferedIterator fills it buffer asynchronously by calling setImmediate: https://github.com/RubenVerborgh/AsyncIterator/blob/251911af831643c7a4fbef7e9ab743b1ee54a12c/asynciterator.js#L862-L866

• Minimise switching from readable = false to readable = true
  When an iterator is set to readable, an event is emitted with a setImmediate: https://github.com/RubenVerborgh/AsyncIterator/blob/251911af831643c7a4fbef7e9ab743b1ee54a12c/asynciterator.js#L282-L295
  Intensive switching from true to false to true to false ... is therefor unfavourable for iterators where a lot of items pass through