PersonIdent: Use java.time instead of older Date and milliseconds

From errorprone: Date has a bad API that leads to bugs; prefer
java.time.Instant or LocalDate.

Replace the long with milliseconds and int with minutes offset with an
Instant and a ZoneOffset. Create new constructors and deprecate
variants with Date, milliseconds and minute offsets.

When comparing instances of PersonIdent truncate the timestamp precision
to 1 second since git commit timestamps are persisted with 1 second
precision [1].

[1] https://git-scm.com/docs/git-commit#Documentation/git-commit.txt-Gitinternalformat

Change-Id: Id4ba1f108e1ba0bfcdd87ba37c67e2d3cc7d254f