Switch `ws-kalive-ms` from fixed interval to sliding

[ptaoussanis]

Sente has a keep-alive mechanism for both the client and server, controlled by the :ws-kalive-ms options.

These are intended to provide an upper bound on idle time, but the current design does not actually achieve this.

The current design:

• Every ws-kalive-ms (fixed interval) check if there's been any activity in the previous window
• If yes: noop
• If no: send a ping

The problem:

When a pong response is received, it'll be in the next window - often very shortly after the ping was sent. This can lead to unintentionally long periods of connection idleness, e.g.:

Period1 (t0+Δ->t1): no activity           => PING at t1 (t0+1Δ) <-
Period2 (t1+Δ->t2): pong received at t1+ε => noop at t2 (t0+2Δ)
Period3 (t2+Δ->t3): no activity           => PING at t3 (t0+3Δ) <-
Period4 (t3+Δ->t4): pong received at t3+ε => noop at t4 (t0+4Δ)
Period5 (t4+Δ->t5): no activity           => PING at t5 (t0+5Δ) <-

The part I'd foolishly missed: if the last activity was the pong at t1+ε, then the interval t3 - (t1+ε) approaches 2Δ (i.e. twice the desired upper bound).

The most obvious solution would be to switch to a sliding window rather than a fixed interval - but there may also be alternative (simpler?) solutions. Most simply probably just halving the default ws-kalive-ms and documenting the current behaviour - but let's evaluate our options.

PRs or ideas welcome, otherwise I'll take a look at doing this myself next time I'm on batched Sente work 👍

Big thanks to @p-himik for identifying this bug.