constructor () {
    this.pendingRequests = []
  }
A queue (FIFO) to hold pipelined requests and link metadata to them as the response is received from the server.
This facilitates the handling of responses when the HTTP requests are pipelined.
A queue is the best structure for this because the sever reponses are provided in the same order as the requests
Cf. https://en.wikipedia.org/wiki/HTTP_pipelining

/!\ it's up to you as a user to ensure that the queue is populated if using
the functionality. This implementation will fail silently if e.g. you try to
call any function accessing the queue while it's empty.