Do not send checkpoint if node is syncing

[goran-ethernal]

Description

A user of Edge had an issue where he was testing Disaster Recovery simulation, where he stopped a validator node, and deleted all its data, to see if the node will sync up, recover, and rejoin consensus.

He noticed that while syncing, node was trying to send checkpoints on epoch ending blocks where he was a minter before it was shutdown and its data got deleted.

Since those checkpoints were already sent to rootchain, this PR introduces a fix, where on submitCheckpoint, it checks if given checkpoint was already sent, and simply returns if it is.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually