Summary:
Move definition and implementation for ArenaWrappedDBIter into its own .h/.cc files. Also, change inlining of functions to better comply with the Google C++ style guide.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5801

Test Plan: make check

Differential Revision: D17371012

Pulled By: anand1976

fbshipit-source-id: c1361abc2851575111e357a63d88be3b3d6cb341