Summary:
Currently users have no idea a key is add, delete or merge from TablePropertiesCollector call back. Add a new function to add it.

Also refactor the codes so that
(1) make table property collector and internal table property collector two separate data structures with the later one now exposed
(2) table builders only receive internal table properties

Test Plan: Add cases in table_properties_collector_test to cover both of old and new ways of using TablePropertiesCollector.

Reviewers: yhchiang, igor.sugak, rven, igor

Reviewed By: rven, igor

Subscribers: meyering, yoshinorim, maykov, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D35373