Make connPoolStats useful

The connPoolStats command is currently broken for a variety of reasons:

• The connection pool stats type has a broken add stats for host command (it only records stats for a host the first time the function is called, rather than accumulating)
• The nia connection pool doesn't provide any accounting for connections in setup or refreshing
• We can't actually see how many connections any particular pool has.

In order to better diagnose the more complex connection pooling model we're currently dealing with, we should fix the bugs in connPoolStats and consider providing stat breakdowns by pool and host.