Replace remove() with setting to null

[ianswett]

Fixes #2569

[martinthomson]

On line 1098, where it says:

  if (sent_packets[pn_space][ack.largest_acked] &&
      IncludesAckEliciting(newly_acked_packets)):
    latest_rtt =

Should you instead be really clear and say:

  if (sent_packets[pn_space][ack.largest_acked] != null &&
      IncludesAckEliciting(newly_acked_packets)):
    latest_rtt =

[mikkelfj]

It's a matter of taste, but I don't really like setting values to null when null is not defined value in a set. Removing something from a set is mathematically well defined. That Javascript happens to be weird doesn't have to affect a specification.

[ianswett]

@mikkelfj I was happy with remove() as well but two issues were filed that both seemed to indicate that using remove() was confusing. I'm open to other resolutions as well(or doing nothing).

[martinthomson]

I personally would prefer to keep remove(x) and use something like contains(x) on the test, but I trust Ian's judgment.

[mikkelfj]

I suspect the issues are about operational behaviour more than formalism, and a null test is just a proposed solution. If so, I think remove and contains is a much better approach.

[ianswett]

I created a 1-line PR #3015 to use contains(). I slightly prefer that one to this one.

[janaiyengar]

I like #3015 better.