This PR adds the following:

• Support for the TLS 1.2 record layer.
• ptls_import function for instantiating a picotls connection object from a set of serialized parameters. At the moment, this function supports TLS 1.2 only.
• ptls_export function for extracting properties of a TLS connection so that it can be imported. At the moment, this function supports TLS 1.2 only.
• ptls_build_tls12_export_params for constructing a serialized set of properties of a TLS 1.2 connection, so that it can be imported to picotls.

The idea is:

• Once the TLS 1.2 handshake is complete, extract the necessary properties (e.g., SecurityParameters) from OpenSSL, and use that to create a picotls connection object, then use picotls for exchanging application data and alerts. The benefit that we can gain from all the performance improvements that are only available in picotls (e.g., non-temporal aes-gcm engine #384).
• Design the API to support import-export in serialized form, so that we can extend the implementation for moving TLS states around.