Simplify GetCertificate to reload certificate on each request, remove maybeReload and goroutine in newKeypairReloader

[chaospuppy]

Summary

Currently, we are using tls.Config.GetCertificate() to retrieve the certificate from the cert property of keypairReloader. Additionally, we're running a goroutine as part of newKeypairReloader that waits 60 seconds before updating the cert property of keypairReloader.

Because GetCertificate is called upon each ClientHello, this extra gorountine and timer represents an unnecessary step in the process and should be removed, with the process of loading the configured TLS keypair into a keypair object moved into maybeReload, which itself should be called in the function returned by GetCertificate.