Add overlapping option to flatMap

[bpinto]

This will cause the next stream to be added before any old streams are removed.

const overlapping = true // or false

const channelA = Kefir.stream((emitter) => {
  console.log('connect a');
  let count = 0, id = setInterval(() => emitter.value(count++), 250);
  return () => { console.log('disconnect a'); clearInterval(id); };
});

const channelB = Kefir.stream((emitter) => {
  console.log('connect b');
  let count = 0, id = setInterval(() => emitter.value(count++), 250);
  return () => { console.log('disconnect b'); clearInterval(id); };
});

const data = {
  a: channelA,
  b: Kefir.combine([channelA, channelB]),
  c: channelB,
};

Kefir.sequentially(1000, ['a', 'b', 'c', undefined])
  .flatMapLatest(p => p ? data[p] : Kefir.never(), { overlapping })
  .log('result');

With overlapping option disabled (default):

connect a
result <value> 0
result <value> 1
result <value> 2
disconnect a
connect a
connect b
result <value> [ 0, 0 ]
result <value> [ 1, 0 ]
result <value> [ 1, 1 ]
result <value> [ 2, 1 ]
result <value> [ 2, 2 ]
disconnect a
disconnect b
connect b
result <value> 0
result <value> 1
result <value> 2
disconnect b
result <end>

With overlapping option enabled:

connect a
result <value> 0
result <value> 1
result <value> 2
connect b
result <value> [ 3, 0 ]
result <value> [ 4, 0 ]
result <value> [ 4, 1 ]
result <value> [ 5, 1 ]
result <value> [ 5, 2 ]
result <value> [ 6, 2 ]
disconnect a
result <value> 3
result <value> 4
result <value> 5
result <value> 6
disconnect b
result <end>

Closes: #235 #236

[bpinto]

This is my first time diving into kefir codebase so I most likely did something silly while trying to implement this feature. In any case, please let me know what you think!

[bpinto]

If only we had object rest spread... 😬

[bpinto]

All these "should add to the queue" tests are kinda silly because I can't really confirm that the queue has changed... not sure how to improve these tests.

[bpinto]

I don't like passing a boolean, not sure about performance impact, but wouldn't it be better to have two methods?

_removeCur and _removeCurWithDelayedUnsubscription