[CF] Rethinking ColumnFamilyHandle and fix to dropping column families

Summary:
The change to the public behavior:
* When opening a DB or creating new column family client gets a ColumnFamilyHandle.
* As long as column family handle is alive, client can do whatever he wants with it, even drop it
* Dropped column family can still be read from (using the column family handle)
* Added a new call CloseColumnFamily(). Client has to close all column families that he has opened before deleting the DB
* As soon as column family is closed, any calls to DB using that column family handle will fail (also any outstanding calls)

Internally:
* Ref-counting ColumnFamilyData
* New thread-safety for ColumnFamilySet
* Dropped column families are now completely dropped and their memory cleaned-up

Test Plan: added some tests to column_family_test

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16101

table/filter_block.h

@@ -46,6 +46,9 @@ class FilterBlockBuilder {
   bool SamePrefix(const Slice &key1, const Slice &key2) const;
   void GenerateFilter();
 
+  // important: all of these might point to invalid addresses
+  // at the time of destruction of this filter block. destructor
+  // should NOT dereference them.
   const FilterPolicy* policy_;
   const SliceTransform* prefix_extractor_;
   bool whole_key_filtering_;