use the mint default cookie protector to generate cookies

[marten-seemann]

Fixes #966. Should be merged after bifurcation/mint#141 gets merged.

[marten-seemann]

@lucas-clemente: We renamed mint.CookieSource to mint.CookieProtector before bifurcation/mint#141 was merged. I made the necessary changes to this PR, and also renamed our variables from source to protector.

[codecov-io]

Codecov Report

Merging #978 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   90.71%   90.85%   +0.14%     
==========================================
  Files          99       98       -1     
  Lines        6543     6513      -30     
==========================================
- Hits         5935     5917      -18     
+ Misses        391      385       -6     
+ Partials      217      211       -6

Impacted Files	Coverage Δ
internal/handshake/cookie_generator.go	90.24% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 851b44c...7ba613c. Read the comment docs.

[raininja]

I'm running into an issue with this code in caddy. Trying to build caddy results in this error:

# github.com/mholt/caddy/vendor/github.com/lucas-clemente/quic-go/internal/handshake
vendor/github.com/lucas-clemente/quic-go/internal/handshake/cookie_generator.go:32:18: undefined: mint.CookieProtector