optimize http file sending

optimize http file sending

This adapts the approach from the single-packet-per-poll-loop improvement
to sending more packets while the socket can take them.

It still falls back to the multi-state scheme if the socket ever chokes,
which it certainly will on larger files, so it's safe while being highly
efficient at smaller file sizes.

Nor should it significantly add to latency for other sockets, it simply
stuffs the pipe asynchronously as much as the pipe can take.

We also increase the packet payoad size from 512 to 1400 a time.

This reduces the time taken in the 300 connection / 5000 transfers ab test
from >8s to ~3.4s, transferring the same amount of data.

$ ab -t 100 -n 5000 -c 300 'http://127.0.0.1:7681/'
This is ApacheBench, Version 2.3 <$Revision: 1373084 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient)
Completed 500 requests
Completed 1000 requests
Completed 1500 requests
Completed 2000 requests
Completed 2500 requests
Completed 3000 requests
Completed 3500 requests
Completed 4000 requests
Completed 4500 requests
Completed 5000 requests
Finished 5000 requests

Server Software:        libwebsockets
Server Hostname:        127.0.0.1
Server Port:            7681

Document Path:          /
Document Length:        8447 bytes

Concurrency Level:      300
Time taken for tests:   3.400 seconds
Complete requests:      5000
Failed requests:        0
Write errors:           0
Total transferred:      42680000 bytes
HTML transferred:       42235000 bytes
Requests per second:    1470.76 [#/sec] (mean)
Time per request:       203.976 [ms] (mean)
Time per request:       0.680 [ms] (mean, across all concurrent requests)
Transfer rate:          12260.17 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        7   24  15.6     20     125
Processing:    32  172  50.2    161     407
Waiting:       27  154  49.4    142     386
Total:         81  196  48.3    182     428

Percentage of the requests served within a certain time (ms)
  50%    182
  66%    185
  75%    188
  80%    194
  90%    304
  95%    316
  98%    322
  99%    328
 100%    428 (longest request)

Signed-off-by: Andy Green <andy.green@linaro.org>