Summary:
My compiler doesn't inline DBIter::Next() to arena wrapped iterator, even if it is a direct forward. Adding this annotation makes it inlined. It might not always work but inlinging this function to arena wrapped iterator always feels like the right decision.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5217

Differential Revision: D15004086

Pulled By: siying

fbshipit-source-id: a4cffd79c6fb092669a3a90633c9aa5e494f8a66