Move timeutils functions to the only places where they are used.

- Move time json marshaling to the jsonlog package: this is a docker
  internal hack that we should not promote as a library.
- Move Timestamp encoding/decoding functions to the API types: This is
  only used there. It could be a standalone library but I don't this
it's worth having a separated repo for this. It could introduce more
complexity than it solves.

Signed-off-by: David Calavera <david.calavera@gmail.com>