Make partial dates without year to be 1970 based instead of 2000

Fixes #4451

Date fields without date (HH:mm:ss, for example) are parsed as time on Jan 1, 1970 UTC. However, before this change partial dates without year (MMM dd HH:mm:ss, for example) were parsed as as days of they year 2000. This change makes all partial dates to be treated based on year 1970. This is breaking change - before this change "Dec 15, 10:00:00" in most cases was parsed (and indexed) as "2000-12-15T10:00:00Z". After this change, it will be consistently parsed and indexed as  "1970-12-15T10:00:00Z"