fix `WriteBufferManager` flush log message (#6335)

Summary:
It chooses the oldest memtable, not the largest one. This is an
important difference for users whose CFs receive non-uniform write
rates.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6335

Differential Revision: D19588865

Pulled By: maysamyabandeh

fbshipit-source-id: 62ad4325b0182f5f27858584cd73fd5978fb2cec

db/db_impl/db_impl_write.cc

@@ -1292,7 +1292,7 @@ Status DBImpl::HandleWriteBufferFull(WriteContext* write_context) {
   // suboptimal but still correct.
   ROCKS_LOG_INFO(
       immutable_db_options_.info_log,
-      "Flushing column family with largest mem table size. Write buffer is "
+      "Flushing column family with oldest memtable entry. Write buffer is "
       "using %" ROCKSDB_PRIszt " bytes out of a total of %" ROCKSDB_PRIszt ".",
       write_buffer_manager_->memory_usage(),
       write_buffer_manager_->buffer_size());