* disable debug printing again
* add a keepalive flag when creating a http request
start implementing the various combination of per connection and global
limits for controlling how many requests are being issued.

These are all currently static limits and there are a few things that
are missing (like timeouts per request, timeouts per connection,
maximum connection lifetime in seconds or bytes, maximum test lifetime,
etc.)

This is making good progress.
* Add the conn hook when creating the conn - that way we get notified if there
  was an actual connect error.
* Annoyingly we don't get told what the connect error is. Grr.
* make sure 'r' exists before dereferencing it when tearing things down.
* bump up the connection rates / limits.
Track number of closing connections.  We don't know why yet - grr.