Motivation:
`SimpleChannelPool` subclasses are likely to override the `connectChannel` method, and are likely to clobber the cloned `Bootstrap` handler in the process. To allow subclasses to properly notify the pool listener of new connections, we should expose (at least) the `handler` property of the pool to subclasses.
Modifications:
Expose `SimpleChannelPool` properties to subclasses via `protected` getters.
Result:
Subclasses can now use the bootstrap, handler, health checker, and health-check-on-release preoperties from their superclass.
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