** (WIP) ** Sync pool operations and eliminate race conditions

This spike re-implements Pool storage using `YAML::Store` from the Ruby stdlib.
This class provides a hash with autoflushing file-backed storage. Concurrent
operations between multiple processes are protected by `File.flock` (to the
extent that `flock` works on a given filesystem).

This is a work in progress and is currently not compatible with the pool files
laid down by 0.2.x or master.