Summary:
Sometimes we iterate through column families, and unlock the mutex in the body of the iteration. While mutex is unlocked, some column family might be created or dropped. We need to be able to continue iterating through column families even though our current column family got dropped.

This diff implements circular linked lists that connect all column families. It then uses the link list to enable iterating through linked lists. Even if the column family is dropped, its next_ pointer still can be used to advance to another alive column family.

Test Plan: make check

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15603

Summary: Making room for write will be the hardest part of the column family implementation. For now, I just iterate through all column families and run MakeRoomForWrite() for every one.

Test Plan: make check does not complain

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15597

Summary: ColumnFamilyData grew a lot, there's much more data that it holds now. It makes more sense to encapsulate it better by making it a class.

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15579

Summary: This one is big. It adds ability to write to and read from different column families (see the unit test). It also supports recovery of different column families from log, which was the hardest part to reason about. We need to make sure to never delete the log file which has unflushed data from any column family. To support that, I added another concept, which is versions_->MinLogNumber()

Test Plan: Added a unit test in column_family_test

Reviewers: dhruba, haobo, sdong, kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15537

Summary: All memtables and immutable memtables are moved from DBImpl to ColumnFamilyData. For now, they are all referenced from default column family in DBImpl. It shouldn't be hard to get them from custom column family.

Test Plan: make check

Reviewers: dhruba, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15459

Summary:
I created a separate class ColumnFamilySet to keep track of column families. Before we did this in VersionSet and I believe this approach is cleaner.

Let me know if you have any comments. I will commit tomorrow.

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15357