Handle leap seconds in date-time format properties (occurred_at)

[a1exsh]

Currently, Nakadi cannot handle leap seconds in the metadata's occurred_at field. However, the internet date and time RFC allows them: https://tools.ietf.org/html/rfc3339#section-5.7

The API should handle leap seconds, for example by shifting them back to ":59:59", like suggested here: https://gist.github.com/dehora/9b2ce8bac22d1b37393d8b4f7c9eb7aa