Avoid not needed renames in ALTER TABLE

Removed not needed table renames when doing ALTER TABLE when engine
changes and both of the following is true:
- Either new or old engine does not store the table in files
- Neither old or new engine uses files from another engine

We also skip renames when ALTER TABLE does an explicit rename

This improves performance, especially for engines where rename is
a slow operation (like the upcoming S3 engine)

0 files changed, 0 insertions, 0 deletions