make it possible to use multiple qtls versions at the same time, add support for Go 1.15

[marten-seemann]

Fixes #2614. Obviously, I need to cut a release of qtls-go115 (and rename the repository, since I accidentally pushed old tags there, and probably polluted the Go module cache with those...) before we can merge this PR. Other than that, this PR should be done.

Due to the super-tight coupling between qtls and tls we need to be able to use multiple versions of qtls, depending on the Go version we're using. This is necessary because we need to do what would be called a reinterpret-cast between structs defined in qtls and in crypto/tls, e.g.

var tlsCert *tls.Certificate
var qtlsCert *qtls.Certificate = (*qtls.Certificate)(unsafe.Pointer(tlsCert))

This is obviously unsafe, but should be fine as long as the struct defined in qtls exactly matches the struct defined in crypto/tls.
When Go updates the struct in crypto/tls (as has happened on multiple occasions both for the Go 1.14 and Go 1.15 release), this violates this assumption. We'd need to include multiple versions of qtls. Unfortunately, Go Modules doesn't allow us to do this. We have to decide for one version.
The workaround around this restriction is as straightforward as it is ugly: use separate repositories for the qtls code for Go 1.14 and Go 1.15. Then all we need is a thin shim layer around qtls, which imports the respective version for Go 1.14 and Go 1.15.

qtls-go115 goes a few steps further than the Go 1.14 version, in that it defines even more type aliases, especially for the tls.Config. This means that as a tls.Config is now essentially as tls.Config. This allows us to get rid of a lot of conversion code.

[codecov]

Codecov Report

Merging #2720 into master will decrease coverage by 0.27%.
The diff coverage is 74.75%.

@@            Coverage Diff             @@
##           master    #2720      +/-   ##
==========================================
- Coverage   86.75%   86.49%   -0.27%     
==========================================
  Files         124      126       +2     
  Lines        9895     9952      +57     
==========================================
+ Hits         8584     8607      +23     
- Misses        979     1013      +34     
  Partials      332      332              

Impacted Files	Coverage Δ
internal/handshake/aead.go	96.36% <ø> (ø)
internal/handshake/initial_aead.go	100.00% <ø> (ø)
internal/handshake/tls_extension_handler.go	100.00% <ø> (ø)
internal/handshake/updatable_aead.go	91.34% <ø> (ø)
internal/qerr/error_codes.go	100.00% <ø> (ø)
internal/qtls/go114.go	36.36% <36.36%> (ø)
internal/qtls/go115.go	36.36% <36.36%> (ø)
internal/handshake/crypto_setup.go	67.20% <77.42%> (+0.25%)	⬆️
internal/qtls/qtls.go	85.96% <85.96%> (ø)
internal/handshake/header_protector.go	83.05% <100.00%> (ø)
... and 5 more

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