Transaction confirmation level flags

[mikera]

When sending a transaction, users may wish to specify when they wish to receive notification regarding the transaction's status.

Options:

• Received - peer has the transaction and promises to submit it
• Submitted - transaction has been included in a block and published
• Proposed - transaction has been proposed for consensus. Speculative result may be available
• Confirmed - transaction has been confirmed in consensus. Final result will be available
• Verified - some further verification of the result, perhaps with multiple peers?

Should be possible for a user to specify what levels types of verification they require.