Change net_send_queue() in preparation for improving send performance.

No longer takes callbacks, flags, or *out arguments.

Update rest of the code that called these callbacks whenever sending
was completed, instead call them right away now.
Chain our sending netbufs together. Gives us a lot less SSL_write() calls.
Improve on sending data to the client.

- Attempt to chain as much as we can in the send netbufs
  (keeps down the SSL_write calls, silly seeing it go out with 8 bytes)
- Change NETBUF_SEND_PAYLOAD_MAX to 4K
- Call SSL_write() with max NETBUF_SEND_PAYLOAD_MAX in size
- Go back to flushing the send buffers after each request
- No more need for a fixed pool for nb->buf, go back to the heap for now
- Disable Nagle, we're doing the chaining now anyway
Kill unused vars