Responses are usually added to the response queue by worker threads. As an
optimization, and in order to close connections as quickly as possible, allow
for immediate replies (bypassing the queue) when there is no need for a worker
to service the request.

No functional change, just preparation for the next commits.

Rather than potentially accepting an unbounded number of connections, give them
a ceiling of twice the size of the processing queue.

Once the ceiling is reached, new connections will not be accepted until the
count has fallen back under the limit.

Note that file descriptors are removed from the limiter in the request's
close callback.

If we're unable to service a request, close the connection rather than
potentially keeping it alive longer. This helps to be slightly more fair to new
connections.

This avoids assigning file descriptors faster than we can close them.

It is the same as using an evconnlistener with the LEV_OPT_DEFERRED_ACCEPT
flag, which is only available with libevent 2.1+.

Libevent 2.2 adds a callback allowing us to drop connections as they come in.
Use it when possible.