Utxos in a successful /transaction.submit but not yet mined should be excluded from account's available utxos

[unnawut]

Copying over @pnowosie's explanation:

• tx submitted to ch-ch
• ch-ch publishes new block's hash in transaction on Ethereum
• Watcher discovers the above Ethereum transaction
• Watcher asks ch-ch of block for a given hash
• Ch-ch responds with block which contains tx
• Watcher processes block and includes tx in DB (here Watcher is Info-Api Watcher)

Latency:

• Time needed for a block to be formed (~1 plasma block per Ethereum block)
• Block publication transaction needs to be mined on Ethereum (depends on gas price and Ehereum blocks freq)
• Then on Watcher side Ehereum event listener check interval
• Network latency in Watcher -> Ch-ch
• Block processing time (checking especially all signatures in large blocks takes a little)
• DB insert

Because of this latency, the same utxo might be picked up by /transaction.create again before it is marked as spent by the first submission.

Need a way to keep track of utxos that are being processed.

[boolafish]

This is a bit tricky though...because only the specific watcher that receives the /transaction.submit request would know such account related info. I think ideally it should be the responsibility of a layer on top of the watcher (eg. e-wallet) to handle account-related states, especially there are pending states.