Add Persistent#max_requests to prevent ECONNRESET

 * Many servers have a max_requests tunable on keepalive connections that is
   tuned to less than infinite. Some of these servers incorrectly close the
   connection out-of-band (rather than replying with Connection:close). In these
   cases, it is advantageous to avoid the resulting ECONNRESET, and preemptively
   reset those connections.
 * Additional potential work that could be done later: Make a closing request on
   the last request. Add a detect_max_requests helper at the top level.
 * Exmaple configuration link:
   http://wiki.nginx.org/HttpCoreModule#keepalive_requests
Merge pull request #42 from raggi/max_requests

Add Persistent#max_requests to prevent ECONNRESET