Serialization - version control & python API (#1491)
Serialization - version control & python API (#1491)

* add version control

* version support

* add python api

* add version control proto

* add unit test

* change function name

* rename function api

* per comments

* refinement to make hierachy more cleaner

* backward compatibility

intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL