Summary:
Existing implementation of PerfContext does not define copy constructor or assignment operator, which could potentially cause problems when user create copies and resets the builtin one. This PR address the issue by providing these two constructors with deep copy semantics.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4919

Differential Revision: D13960406

Pulled By: miasantreble

fbshipit-source-id: 36aab5aaee65d4480f537e4e22148faa45e8e334