Add a GetComparator() function to the ColumnFamilyHandle base class so that the user's comparator can be retrieved.

Summary: MyRocks is adding support for the user of the SstFileWriter which needs a comparator.  It would be more convenient to get the comparator from the column family (which already has to have it) than to have caller keep track of it.

Test Plan: Standard tests (adding one for the new method)

Reviewers: IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61155