Check if ETH node is synced on startup

[yondonfu]

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A user may try connecting the livepeer node to an Ethereum node that is not fully synced - this can cause issues such as the code for the livepeer contracts not existing.

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The livepeer node can use the eth_syncing RPC call on startup to check if the Ethereum node it is connected to is fully synced. If the Ethereum node is not fully synced, there are a few options:

• Wait for the Ethereum node to be fully synced
• Exit early and tell the user that the Ethereum node is not fully synced and to try running livepeer again when the Ethereum node is fully synced

I think something along the lines of the latter is probably better since in the former the Ethereum node could take awhile to sync.

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Additional context
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A user in Discord tried connecting to an Ethereum node that was not fully synced and encountered:

E0709 06:18:18.756417    3944 client.go:222] Error getting LivepeerToken address: no contract code at given address
E0709 06:18:18.758606    3944 livepeer.go:378] Failed to setup client: no contract code at given address
I0709 06:18:18.761430    3944 db.go:381] Closing DB