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use TLS 1.3

TLS 1.3 support was recently made opt-in in Go 1.12, so we need to
explicitly enable it.

TLS 1.3 will require Go >= 1.12, so we don't need any fixes that were
specific to Go 1.10 any more.

remove unneeded marshaling / unmarshaling when generating cert chain

avoid using interface{} when generating certificates

add support for ECDSA keys

close the underlying connection when the context is canceled

add gx, Travis and Codecov configuration