[CF] Dont reuse dropped column family IDs

Summary:
Column family IDs should be unique, even if column family is dropped. To achieve this, we save max column family in manifest.

Note that the diff is still not ready. I'm only using differential to move the patch to my Mac machine.

Test Plan: added a test to column_family_test

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16581

db/version_edit_test.cc

@@ -49,6 +49,7 @@ TEST(VersionEditTest, ColumnFamilyTest) {
   VersionEdit edit;
   edit.SetColumnFamily(2);
   edit.AddColumnFamily("column_family");
+  edit.SetMaxColumnFamily(5);
   TestEncodeDecode(edit);
 
   edit.Clear();