perf(ext/http): optimize away code based on callback length

[littledivy]

hello world on macOS:

divy@mini ~> wrk -d 10s --latency http://127.0.0.1:4500
Running 10s test @ http://127.0.0.1:4500
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    80.82us   42.95us   2.91ms   96.40%
    Req/Sec    56.91k     1.94k   60.77k    95.54%
  Latency Distribution
     50%   77.00us
     75%   89.00us
     90%  105.00us
     99%  146.00us
  1143455 requests in 10.10s, 138.49MB read
Requests/sec: 113212.38
Transfer/sec:     13.71MB

divy@mini ~> wrk -d 10s --latency http://127.0.0.1:4500
Running 10s test @ http://127.0.0.1:4500
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    88.63us   78.77us   2.55ms   98.72%
    Req/Sec    54.84k     2.16k   57.35k    98.51%
  Latency Distribution
     50%   80.00us
     75%   93.00us
     90%  109.00us
     99%  249.00us
  1102313 requests in 10.10s, 133.51MB read
Requests/sec: 109136.61
Transfer/sec:     13.22MB

Expected to have a larger impact on Linux

[mmastrac]

LGTM.. we should move remoteAddr to the request itself so we can avoid this check.