add `verifiedAt` timestamp for transactions which have been verified by dcipher

[CluEleSsUK]

can we add executedAt ? We will then have timestamps for requestedAt, fulfilledAt, and executedAt which are going to help us identify any bottlenecks and support claims about 'how fast' we are. Also nice to show execution time to users.

Originally posted by @andrewjslack in randa-mu/onlyswaps-js#13 (comment)