reduce the maximum number of ACK ranges

[marten-seemann]

This is basically a prerequisite for #2885, as we'll keep ACK frames with []wire.AckRange in the buffer.

Imagine the worst-case scenario here: We're receiving every packet with a gap after the last packet (e.g. packet numbers 2, 4, 6, 8, ...). Every out-of-order packet will elicit an ACK frame, so we'll send out every ACK range in 31 ACK frames (assuming we're not getting an ACK back that clears those ranges). This is assuming that we're not processing packets faster than we send out ACKs (the select in the session run loop is not deterministic).

[codecov]

Codecov Report

Merging #2887 (33c7588) into master (69158cf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2887   +/-   ##
=======================================
  Coverage   85.77%   85.77%           
=======================================
  Files         133      133           
  Lines        9187     9187           
=======================================
  Hits         7880     7880           
  Misses        959      959           
  Partials      348      348           

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