(PUP-1411) Increase CLIENT_HELLO timeout

Previously, if a client established a TCP connection to the webrick server,
but the server did not receive any data within 6.2 seconds, the server
would close the connection, and report that the client disconnected.

The error message is actually misleading, because if the client did
actively disconnect, e.g. sent FIN or RST, then IO.select would report that the
socket is ready and timeout would not be reached.

This commit increases the timeout to 10 seconds and updates the error
message to more clearly say that the client did not send any data
before the timeout expired.

Note that we perform this check, because OpenSSL's SSL_accept will block
indefinitely, since the underlying socket is blocking by default.