Cancel stale transactions

As a user, I want old or stale transactions to be canceled so that I don't have a buildup of transactions that are no longer relevant to me.

Automatically cancel transactions when:

1) A transactions is pending for over 12 hours
2) When a new transactions is initiated from a different dapp than any current pending transaction
3) When a user closes the MetaMask confirmation window

This is related to the design work for pending transactions: https://www.figma.com/file/hm2RQXkH2QVRJnnTnArLM7/Confirmations?node-id=656%3A1

And `Reject all pending confirmations when notification window closed #10302`
https://github.com/MetaMask/metamask-extension/issues/10302

Read the linked Figma design and related issue #10302 first, then locate the pending transaction and confirmation-window handling they describe. Done means stale transactions are canceled after 12 hours, when a different dapp starts one, and when the confirmation window closes.

Written by the indexing model from the issue text.