* Rename Puma::Cluster::{Worker => WorkerHandle}
This class represents a worker from the perspective of the puma master
process. It provides methods for controlling the process, but doesn't
contain the logic actually executed by the worker.
In preparation for creating a new class that encapsulates the worker
logic, we're renaming this one to WorkerHandle.
* Extract Puma::Cluster::WorkerHandle to a separate file
* Move worker functionality to new class
Before, all functionality of the worker processes was defined in the
Cluster class. In preparation for making it possible to start worker
processes outside of the context of a Cluster instance, we move the
worker functionality into a new class.
This has the additional benefit of delineating exactly the dependencies
of the worker processes, namely the Launcher, options hash, and the
pipes used for inter-process communication.
* Extract nakayoshi_gc to Puma::Util
* Add comment to describe Puma::Cluster::WorkerHandle
* Remove options from Worker constructor
The instance varaible @options can be derived from the @launcher
