Increase the nginx # worker connections. Increase the # threads ulimit.

Both should allow higher concurrently levels. The nginx
worker_connections is also now configurable via a config file parameter.

In addition, we now attempt to set the ulimits as high as we can for
non-root users. This helps us bump the number of processes ulimit in our
test environment.
Attempt to cleanup keepalive server tests to make more reliable.

The main confusion was that the keepalive number is per nginx worker
process, not global. We've also increased the concurrency to try to
saturate the connections more reliably
(9799f15d1d0495602897e5069a1f41aabf8a01f5 should also help with this to
ensure we're not hitting # of threads/processes limits internally and
lowering the concurrency).