NUT-06: Mint should say what the current time is

[callebtc]

In order for users to be sure that their local clock agrees with the mint's clock, the mint should publish its current time in the info response.

[prusnak]

we should either use Unix timestamp or format returned by new Date().toISOString() which is2012-04-23T18:25:43.511Z (where the time used is UTC), rationale: https://stackoverflow.com/questions/10286204/what-is-the-right-json-date-format

[thesimplekid]

I would prefer the unix timestamp. Its what we use in other places.

[prusnak]

• nutshell PR: implement nut-06 time nutshell#611
• cdk PR: feat: implement nut-06 time cdk#313