Summary:
this avoids a few copies of std::string and other structs
in the context of range-based for loops. instead of copying
the values for each iteration, use a const reference to avoid
copying.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4459

Differential Revision: D10282045

Pulled By: sagar0

fbshipit-source-id: 5012e910dca279abd2be847e1fb432d96274edfb