of outstanding requests on a connection. Set rate limits for request
execution:
   no connection can have more than maxthreads/2 ops executing at once.
   a connection that is write-blocked will not execute any new ops.
   queued ops must drain before any new ops can execute.
If the queue exceeds the max_pending limit, the connection is closed.
...also fixed a bug where a connection was not marked active if it never
received a Bind.