Add Cluster Support for Slaves

Slave mode processes now use the Node cluster module to spin up one or more processes.  We trade memory for CPU: each slave worker needs its own rewind buffers in memory, but we're able to spread across more cores.

Also included: 

* Fix for a handoff case where the last listener queued for sending disconnects before the send happens.

* Better logic for opening w3c log files. Fix for a bug where log files wouldn't re-open if they already existed.

* New Console logger with support for ignoring certain log levels.