serialize frames by appending to a byte slice, not to a bytes.Buffer

[marten-seemann]

The allocation of the bytes.Buffer in the packet packer causes ~32 MB of allocations for sending of 1 GB of application data:
https://github.com/lucas-clemente/quic-go/blob/07412be8a02ef0e55580ebf8db9c38a759c0a0e5/packet_packer.go#L828

Instead of allocating to a bytes.Buffer, we can just append to a []byte.

This PR doesn't remove the allocation of the bytes.Buffer, since the header serializer still needs it. As part of the effort to refactor our header handling, we'll change this, at least for Short Header packets.

[codecov]

Codecov Report

Base: 85.61% // Head: 85.66% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (ab6d664) compared to base (07412be).
Patch coverage: 95.16% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3530      +/-   ##
==========================================
+ Coverage   85.61%   85.66%   +0.05%     
==========================================
  Files         137      137              
  Lines       10016    10029      +13     
==========================================
+ Hits         8575     8591      +16     
+ Misses       1065     1063       -2     
+ Partials      376      375       -1     

Impacted Files	Coverage Δ
packet_packer.go	84.55% <63.64%> (+0.09%)	⬆️
server.go	81.16% <80.00%> (+0.05%)	⬆️
internal/wire/new_connection_id_frame.go	94.59% <87.50%> (ø)
internal/wire/ack_frame.go	94.78% <100.00%> (ø)
internal/wire/connection_close_frame.go	94.74% <100.00%> (ø)
internal/wire/crypto_frame.go	95.45% <100.00%> (ø)
internal/wire/data_blocked_frame.go	100.00% <100.00%> (ø)
internal/wire/datagram_frame.go	95.12% <100.00%> (ø)
internal/wire/handshake_done_frame.go	100.00% <100.00%> (+50.00%)	⬆️
internal/wire/max_data_frame.go	100.00% <100.00%> (ø)
... and 14 more

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