datetime: Mons intervals

Split single sec field in the datetime_interval into multiple subfields,
i.e. week, day, hour, min, sec. Make sure that whatever we enter in the
constructor we get similarly visualized as textual representation

Lua implementation of datetime_totable has been converted to C, which
allows to use it for datetime values decomposing in datetimes
subtraction. It uses per component subtraction for calculating of a
resultant interval.

Part of #6923

NO_DOC=refactoring
NO_CHANGELOG=refactoring