Support submitting multiple transactions at once

[AlicanC]

When submitting multiple transactions at once, there's a risk that same coins get included in more than one txn, causing some txns to fail.

[AlicanC]

We could keep a list of pending coins in Wallet, and use that list for exclusion when calling coinsToSpend after the following is implemented: FuelLabs/fuel-core#185

[luizstacio]

This would require some storage management to keep the queue safe from accidental termination, such as the exit process on node.js or closed browser?

[AlicanC]

I think we can regenerate all the data we need by querying pending transactions.

[Torres-ssf]

Closing this as we already have multi call working