tls::set_verify_callback(), which set_server_certificate_verifier() uses,
stored the callback in a single thread_local slot shared by every context.
A verifier was skipped when the handshake ran on another thread, ran for
other clients on the thread that set it, and could run after its client
was destroyed.
Store it with the context instead: in SSL_CTX ex_data for OpenSSL, and in
MbedTlsContext and WolfSSLContext for Mbed TLS and wolfSSL.
yhirose
6fc677444f
