Optimizations for immediate write code paths

Release 1.17.15

- [BUGFIX] http_client: make sure only one read per on_read() callback
  is performed in the header conversion bypass (-B) mode.
- http_client: with -E, assign random priority when stream is created.
- [OPTIMIZATION] On immediate write, place an ACK frame into the first
  buffered packet if an ACK is queued.  This reduces the number of
  standalone ACK packets.
- [OPTIMIZATION] Allow placing more than one STREAM frame from the same
  stream into an outgoing packet.  This change minimizes the number of
  buffered packets required to store several small HTTP messages by
  virtue of allowing more than one STREAM frame from HEADERS stream in
  the same packet.
- [OPTIMIZATION] Flush headers when writing to buffered packets.  This
  causes the headers to be written to the same buffered packet queue,
  thereby improving packet utilization, especially for small HTTP
  messages.