perf_context.get_from_output_files_time is set for MultiGet() and ReadOnly DB too.

Summary: perf_context.get_from_output_files_time is now only set writable DB's DB::Get(). Extend it to MultiGet() and read only DB.

Test Plan:
make all check
Fix perf_context_test and extend it to cover MultiGet(), as long as read-only DB. Run it and watch the results

Reviewers: ljin, yhchiang, igor

Reviewed By: igor

Subscribers: rven, leveldb

Differential Revision: https://reviews.facebook.net/D24207