Various downstream writing fixes.

* bugfix: ngx.flush(), ngx.eof(), and some other things did not update
  busy/free chains after calling the output filters.
* bugfix: ngx_gzip/ngx_gunzip filters might cause ngx.flush(true) to
  hang until timeout for nginx 1.7.7+ (and some other very old versions
  of nginx). thanks Maxim Dounin for the help.
feature: added nginx configuration file names and line numbers to the rewrite/access/content/log_by_lua directives' Lua chunk names in order to simplify debugging.
Various improvements in ngx.timer and fake connections/requests.

* optimize: reduced the pool size of a fake connection from the
  default pool size (16KB) to 128B.
* optimize: made fake requests share their connection pools.
* feature: the error logger used by ngx.timer.at handlers now
  outputs the "client: xxx, server: xxx" context info for the
  original (true) request creating the timer.
fixed some new test failures according to the Amazon EC2 report.
Merge branch 'master' of github.com:openresty/lua-nginx-module
doc: bumped version to 0.9.13 and also claimed the compatibility with nginx cores up to 1.7.7.