use PADDING frames to pad packets

[marten-seemann]

Fixes #2875.

This is a non-trivial rewrite of how we pack packets. Now we compose the payload of an entire datagram (i.e. multiple coalesced packets), before serializing anything. This allows us to add PADDING frames to the Initial packet.

In order for this to work properly, we need to make sure encode the Length field in the Long Header in 2 bytes (even if the actual length would have in 1 byte).

[codecov]

Codecov Report

Merging #2876 (d1a784d) into master (e676a83) will increase coverage by 0.06%.
The diff coverage is 87.43%.

@@            Coverage Diff             @@
##           master    #2876      +/-   ##
==========================================
+ Coverage   85.71%   85.77%   +0.06%     
==========================================
  Files         134      134              
  Lines        9112     9188      +76     
==========================================
+ Hits         7810     7881      +71     
+ Misses        960      959       -1     
- Partials      342      348       +6     

Impacted Files	Coverage Δ
packet_packer.go	84.45% <85.99%> (+1.04%)	⬆️
internal/utils/varint.go	78.38% <100.00%> (+5.96%)	⬆️
internal/wire/extended_header.go	87.60% <100.00%> (ø)

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[marten-seemann]

Merging, as we need to release a new version asap.