Create a new context when renewing a certificate in the background

[ankon]

The context available to renewDynamicCertificate comes from inside the TLS handshake, and as such may be bounded by the lifespan of the connection. Passing this into a goroutine will lead to problems when the connection ends (and the connection context gets canceled with it) but the goroutine is going to do more I/O on that context.

[ankon]

FWIW, go test -v -short -race ./... fails locally:

=== RUN   TestFindZoneByFqdn/domain_is_a_eTLD
    dnsutil_test.go:172: test 2: expected no error, but got: could not find the start of authority for example.com.ac.: NXDOMAIN
    dnsutil_test.go:175: test 2: expected zone '' but got 'ac.'

[mholt]

Just seeing this -- exactly the change I was going to make after I read the issue.

Thanks for the patch!

[mholt]

FWIW, go test -v -short -race ./... fails locally:

Yeah, the CertMagic tests currently use live DNS which I kind of hate 😅 I just haven't done the work to mock them up yet. They can be problematic depending on which environments you run them in.