Implement KeepAlive for AsyncClient

[davidvonthenen]

Proposed changes

Addresses issue: #272

Changes:

• This implements KeepAlives and Protocol pings for AsyncClient
• Standardizes the ping loop thread between the two clients (allows the thread to exit earlier instead of waiting for longer periods of time)

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update or tests (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc
• I have lint'ed all of my code using repo standards
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

NA