speed up marshaling of transport parameters

[marten-seemann]

This is not in the critical path, so technically this is not a part of #3526. It's fun to see how much we can gain anyway, and such an easy and self-contained change that it's definitely worth doing.

The speedup comes from multiple sources:

1. We now preallocate a byte slice, instead of appending multiple times.
2. Marshaling into a byte slice is faster than using a bytes.Buffer.
3. quicvarint.Write allocates, while quicvarint.Append doesn't.

benchmark                                  old ns/op     new ns/op     delta
BenchmarkTransportParametersMarshal-10     368           196           -46.80%

benchmark                                  old allocs     new allocs     delta
BenchmarkTransportParametersMarshal-10     14             1              -92.86%

benchmark                                  old bytes     new bytes     delta
BenchmarkTransportParametersMarshal-10     273           256           -6.23%

[codecov]

Codecov Report

Base: 85.56% // Head: 85.55% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (9ae9a90) compared to base (3f1adfd).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3531      +/-   ##
==========================================
- Coverage   85.56%   85.55%   -0.00%     
==========================================
  Files         137      137              
  Lines       10087    10086       -1     
==========================================
- Hits         8630     8629       -1     
  Misses       1080     1080              
  Partials      377      377              

Impacted Files	Coverage Δ
internal/handshake/crypto_setup.go	66.74% <100.00%> (-0.08%)	⬇️
internal/handshake/session_ticket.go	100.00% <100.00%> (ø)
internal/wire/transport_parameters.go	89.21% <100.00%> (+0.04%)	⬆️

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