Check if known nonce is higher than pending tx

[bdresser]

If users submit tx outside MetaMask it can cause us to lose track of which tx are actually pending, which can affect which transaction we ask the user to reprice / speed up.

As described in #4974 (comment), we can check on pending tx if the account's known nonce has exceeded the pending transaction in metamask and if so transition the status to dropped.

[rstormsf]

Bump.
I just experienced an issue due to stalled pending txs.
Metamask becames unusuable. What I had is the following:
Send tx with less than acceptable gas price: 0.1 gwei on kovan
Send some more txs on top.
Send tx outside of metamask to drop pending txs.

Result: Metamask still shows old pending tx and it cannot cancel them, the account becomes unusable due to inability to generate new transactions.

[bdresser]

@rstormsf thanks for the bump

as a fix in the meantime, doing a "reset account" from settings should force the extension to re-sync your nonce.