[core] allow up to 32k of data frames per stream

allow up to 32k of data frames per stream per round
(previously limited to single max_frame_size (default 16k))

For 8 streams, 32k*8 is 256k, which is current lighttpd MAX_WRITE_LIMIT,
so each stream still gets a chance to write data (unless write queue
not emptied on previous attempt, reducing add limit this round)

src/connections.c

@@ -1273,7 +1273,7 @@ connection_state_machine_h2 (request_st * const h2r, connection * const con)
                         & (FDEVENT_STREAM_RESPONSE
                           |FDEVENT_STREAM_RESPONSE_BUFMIN)))) {
 
-                uint32_t dlen = (uint32_t)max_bytes;
+                uint32_t dlen = max_bytes > 32768 ? 32768 : (uint32_t)max_bytes;
                 dlen = h2_send_cqdata(r, con, &r->write_queue, dlen);
                 max_bytes -= (off_t)dlen;