Avoid draining pool in gardener cleanup

[wtn]

When a Sync scope exits while the gardener is waiting, Async::Cancel interrupts @condition.wait(@mutex) inside @mutex.synchronize, producing a ThreadError warning. The gardener's ensure also called self.close → drain, which could block if resources are still acquired during teardown.

warn: Async::Task: Async::Pool::Controller Gardener
  Task may have ended with unhandled exception.
    ThreadError: Attempt to unlock a mutex which is not locked
    → async-pool-0.11.2/lib/async/pool/controller.rb:132 in `synchronize'
    Caused by Async::Cancel: Task was cancelled

The gardener's ensure now force-retires resources directly instead of calling close/drain.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.