Format timestamps using ISO 8601 formatting

Summary:
(e.g. "2018-02-28T12:34:56Z")

• Z is the zone designator for the zero UTC offset.
• T is the delimiter used to separate date and time.

This makes it clear for the end-user that the date/time logged is
specified in UTC and not in the local time zone.

Backport of core PR12567

Depends on D2423

Test Plan:

make check

./src/bitcoind

Check in the log the the time format is ISO 8601 compliant

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2425