Flush only one column family

Summary:
Currently DBImpl::Flush() triggers flushes in all column families.
Instead we need to trigger just the column family specified.

Test Plan: make all check

Reviewers: igor, ljin, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20841

db/column_family_test.cc

@@ -913,7 +913,9 @@ TEST(ColumnFamilyTest, DontRollEmptyLogs) {
   }
   int num_writable_file_start = env_->GetNumberOfNewWritableFileCalls();
   // this will trigger the flushes
-  ASSERT_OK(db_->Write(WriteOptions(), nullptr));
+  for (size_t i = 0; i <= 4; ++i) {
+    ASSERT_OK(Flush(i));
+  }
 
   for (int i = 0; i < 4; ++i) {
     dbfull()->TEST_WaitForFlushMemTable(handles_[i]);