QUIC: cleanup send context properly.

QUIC: cleanup send context properly.

The patch resets ctx->frames queue, which may contain frames.  It was possible
that congestion or amplification limits prevented all frames to be sent.

Retransmitted frames could be accounted twice as inflight: first time in
ngx_quic_congestion_lost() called from ngx_quic_resend_frames(), and later
from ngx_quic_discard_ctx().