handshake: test SSL_CTX pool

[npajkovsky]

One option that was not tested, is to also use an SSL_CTX pool of the same size as the libctx pool size, with server and client SSL_CTX objects allocated alongside the associated libctx. Multiple threads, that use the same libctx, might then also use the same SSL_CTX objects. This amortises SSL_CTX creation across not only multiple connections in a thread, but across multiple threads in the same pool.