prevent hot loop when 0rtt is rejected

[kazuho]

At the moment, scheduler_can_send returns 1 if 0-RTT is rejected and there is some application data to be sent.

This results in a hot loop in the applications, as quicly_get_first_timeout would request invocation of quicly_send at an earliest moment. Each time quicly_send is called, zero packets will be emitted, but quicly_get_first_timeout will continue requesting immediate invocation.