Projects each source value to an Observable which is merged in the output Observable, in a serialized fashion waiting for each one to complete before merging the next.
将source Observable里的每个元素施加一个projection函数，这个projection函数返回一个新的Observable，然后将所有这些Observable flatten成另一个Observable.
Returns an Observable that emits items based on applying a function that you supply to each item emitted by the source Observable, where that function returns an (so-called “inner”) Observable. Each new inner Observable is concatenated with the previous inner Observable.
例子：
const clicks = fromEvent(document, 'click');
const ioe = interval(1000);
const mapfn = take(4);
const comp = ioe.pipe(mapfn);
const fn = ev => comp;
const result = clicks.pipe( concatMap(fn));
result.subscribe(x => console.log('diablo: ' +x));
