Increase various limits to eliminate errors

Set most of these config values to 65535 which is pretty big but should be
enough for the high-concurrency plaintext test.

* nginx needs more worker_connections.
* nginx should call listen() with a bigger backlog value.
* uWSGI should call listen() with a bigger backlog value.
* Increase somaxconn (which is a max for the listen() calls).
* tcp_max_syn_backlog seems to act as a max for somaxconn (according to
http://blog.dubbelboer.com/2012/04/09/syn-cookies.html), so increase it.
* Increase max file descriptor limit. For servers this will allow more
concurrent connections and for the wrk client, this will allow 16384
TCP connections plus whatever other file descriptors wrk needs to operate
(such as stdin, stdout, stderr and more).
Merge pull request #543 from MalcolmEvershed/increase-limits

Increase various limits to eliminate errors