Allow CompactFiles and ScheduleCompactFiles to take column family name.

Summary:
Typically, before developers might call CompactFiles / ScheduleCompactFiles,
they will typically get database information via GetDatabaseMetaData() and
OnFlushCompleted().  However, these two functions both use std::string to
refer to a column family instead of ColumnFamilyHandle, while the current
CompactFiles and ScheduleCompactFiles take ColumnFamilyHandle instead.
This introduces one level of indirection to the developers.

This diff allow CompactFiles and ScheduleCompactFiles to take std::string
to specify a column family.

Test Plan:
make compactor_test
./compactor_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22485