Send message out only if it is valid

[boyuan-chen]

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Overview

Describe what your Pull Request is about in a few sentences.

There are two possible ways to fix this invalid payload is shared to other nodes.

1. We can gossip the payload only if it is valid.
2. We can drop the invalid payload if it is from the gossip (unsafe block)

This PR implements the first method and this #188 implements the second method. The first method is more reasonable.

Changes

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• Send out the message only if it is valid

Testing

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[jyellick]

The logic changes seem good to me. I'm a little confused why agossip.Clear() is called in all paths, even on success. But the existing code seems to be doing so.