Summary:
To support column families, it is easiest to use VersionSet to manage
our column families (if we don't have Versions then ColumnFamilyData always
behaves as a dummy column family). This diff only refactors the existing repair
logic to use VersionSet; the next two parts will add support for multiple
column families.

Test Plan:
  $ ./repair_test

Reviewers: yhchiang, IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D59775