Optimize HTTP requests memory usage

Optimize HTTP requests memory usage

Due to historical reasons, ejabberd loads the whole file/data
into the memory when serving an HTTP request. This is now improved:

1) For GET requests ejabberd uses sendfile(2) if the underlying
   connection is HTTP and falls back to read/write loop with 64kb
   buffer for HTTPS connections. This type of requests are handled
   by mod_http_fileserver, mod_http_upload, ejabberd_captcha, etc
2) POST requests are now limited to 20Mb and are fully downloaded
   into the memory for further processing (by ejabberd_web_admin,
   mod_bosh, etc)
3) PUT requests (e.g. for mod_http_upload) are handled by read/write
   loop with 64kb buffer