Summary:
Previously, if you opened a db with num_levels set lower than
the database, you received the unhelpful message "Corruption:
VersionEdit: new-file entry." Now you get a more verbose message
describing the issue.
Also, fix handling of compression_levels (both the run-over-the-end
issue and the memory management of it).
Lastly, unique_ptr'ify a couple of minor calls.
Test Plan: make check
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D8151
0b83a83191