Fix panic in FileStore.walkKeys

If a TSM file is replaced while walkKeys is running, a panic could
occur because the mmap has been unmapped.

tsdb/engine/tsm1/file_store.go

@@ -311,6 +311,12 @@ func (f *FileStore) WalkKeys(seek []byte, fn func(key []byte, typ byte) error) e
 		return nil
 	}
 
+	// Ensure files are not unmapped while we're iterating over them.
+	for _, r := range f.files {
+		r.Ref()
+		defer r.Unref()
+	}
+
 	ki := newMergeKeyIterator(f.files, seek)
 	f.mu.RUnlock()
 	for ki.Next() {