TODO

* add some level of trace to help debugging

* connect error handling

* error handling - eg disconnect

* auto-reconnect

* authentication

* multi-connection pooling

* slave_ok, connect to replica slaves.
  - is auto-upgrade to a master connection when needed the right thing todo? (see ::Database->_exec)

* documentation

* when using prefetch, the next page can arrive before the callback returns if enyevent loop is entered within callback. can/should we guard against it

* tests

* GridFS

* exhaust option to op_query