TLS: concurrent read and write deadlocks

[pgit]

When sending and receiving data concurrently from a TLS stream, neither operation makes progress:

co_await capy::when_all(reader(stream), writer(stream, "response payload"));

Doing both consecutively works:

   std::ignore = co_await writer(stream, "response payload");
   std::ignore = co_await reader(stream);

This is part of a minimal standalone repro made of a client and server that exchange a few bytes:

• https://github.com/pgit/nghttp2-corosio/blob/a10a46e122da61677d1d902a6746c29c30ad1617/tls_repro.cpp#L92

Corosio 31a078b