add the exhaustive linter, replace panics by return values in logging stringers

[marten-seemann]

Depends on #2728.

A while back, @lanzafame suggested to replace the panics in the Stringers of logging types by "unknown" return values. Of course, he was right with that suggestion. Logging is not a good enough reason to panic.

Turns out that there's an even better solution for this: the exhaustive linter, which even is part of the golangci-lint suite. This linter allows us to check that switch statement for enum types cover all values defined for that enum.

There are a few cases in our code where we deliberately don't do an exhaustive check, and I've added a nolint:exhaustive statement there. This is fine, at least it forces us to be explicit about this. In fact, activating this linter actually caught a bug in the packet packer (#2728).

[codecov]

Codecov Report

Merging #2729 into master will increase coverage by 0.01%.
The diff coverage is 73.91%.

@@            Coverage Diff             @@
##           master    #2729      +/-   ##
==========================================
+ Coverage   85.92%   85.93%   +0.01%     
==========================================
  Files         133      133              
  Lines       12075    12087      +12     
==========================================
+ Hits        10375    10386      +11     
- Misses       1365     1366       +1     
  Partials      335      335              

Impacted Files	Coverage Δ
internal/protocol/encryption_level.go	90.91% <ø> (-9.09%)	⬇️
internal/protocol/packet_number.go	100.00% <ø> (ø)
qlog/types.go	85.12% <0.00%> (ø)
packet_packer.go	83.68% <80.00%> (+0.12%)	⬆️
internal/wire/transport_parameters.go	87.38% <92.50%> (-0.08%)	⬇️
crypto_stream_manager.go	90.00% <100.00%> (+0.34%)	⬆️
internal/ackhandler/received_packet_handler.go	77.65% <100.00%> (+0.54%)	⬆️
internal/ackhandler/sent_packet_handler.go	73.66% <100.00%> (+0.04%)	⬆️
internal/handshake/crypto_setup.go	65.21% <100.00%> (+0.07%)	⬆️
internal/wire/extended_header.go	86.83% <100.00%> (+0.16%)	⬆️
... and 4 more

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[lucas-clemente]

GH is showing tons of lint errors. Do you want to fix those now or later?