make RpcClient implement java.io.Closeable

[JHahnHRO]

Proposed Changes

Resolve Issue #1032 by making RpcClient implement java.io.Closeable. The close method was rewritten to be idempotent in accordance with the Spec of Closeable. Now RpcClient can be used in a try-with-resources block and IDEs will (hopefully) remind developers if they forget to call the close method.

Types of Changes

• Bugfix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq) @pivotal-cla This is an Obvious Fix
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

[pivotal-cla]

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[pivotal-cla]

@JHahnHRO This Pull Request contains an obvious fix. Signing the Contributor License Agreement is not necessary.

[michaelklishin]

This is an interesting interpretation of Closeable, and for RpcClient, it makes sense. @acogoluegnes any objections?

[acogoluegnes]

LGTM. Thanks!