Piper/handle already waiting error during bonding

[pipermerriam]

Fixes #737

What was wrong?

Looks like we semi-regularly encounter situations where we're trying to bond with the same peer which causes an AlreadyWaiting exception to bubble.

How was it fixed?

• Catch the AlreadyWaiting exception in bond and return False indicating that bonding failed
• Do a check prior to pinging to check if we're already waiting for a pong.
• Use context manager for managing self.ping_callbacks to ensure the event is always cleared.

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[pipermerriam]

This doesn't actually appear to have solved the problem but I need to investigate more in the morning when I'm fresh.

[pipermerriam]

replaced by #765