Feature/graceful shutdown

[tcnksm]

For hot-deploying, support lestrrat/go-server-starter listeners.

Graceful shutdown is done by Go1.8's graceful shutdown feature (see release note). So this PR must be merged after go1.8 release.

Test

To test hot-deploying feature, run wbt by server-starter and kill HUP to it constantly (per sec.) and send 1000 requests to check there is no lost requests.

To kick wbt,

$ start_server --port 8080 --pid-file=/tmp/test.pid -- wbt -c config/example.toml

To constantly kill HUP it,

$ while true; do kill -HUP $(cat /tmp/test.pid); sleep 1s; done

Requests are done by hey command,

$ hey -n 1000 -c 100 -D test.json -m POST 'http://localhost:8080/wbt'

The results are the followings. Sometimes requests takes long time (but very small number) but no requests are lost.

Summary:
  Total:        25.3181 secs
  Slowest:      19.4102 secs
  Fastest:      0.0004 secs
  Average:      1.6511 secs
  Requests/sec: 39.4974
  Total data:   564877 bytes
  Size/request: 564 bytes

Status code distribution:
  [200] 1000 responses

Response time histogram:
  0.000 [1]     |
  1.941 [623]   |∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  3.882 [259]   |∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  5.823 [86]    |∎∎∎∎∎∎
  7.764 [6]     |
  9.705 [0]     |
  11.646 [0]    |
  13.587 [0]    |
  15.528 [0]    |
  17.469 [0]    |
  19.410 [25]   |∎∎

Latency distribution:
  10% in 0.0029 secs
  25% in 0.0049 secs
  50% in 0.0309 secs
  75% in 1.9808 secs
  90% in 5.0027 secs
  95% in 5.0092 secs
  99% in 19.4014 secs

[cubicdaiya]

@tcnksm Test is still failure.