Summary of changes.

1. Introduce --innodb-directories := "dir1;...;dirN". This is renamed
   from --innodb-scan-directories which was introduced in WL#9499.
   Currently this is not dynamic.
2. Revert to 5.6 behaviour where we scan directories and find .ibd files.
   If you move tablespaces to a new location, add the path to
   innodb-data-directories so that InnoDB can "discover" the files on
   startup.
3. Tablespaces can only be created under known directories. This
   eliminates the need for .isl files.
4. Upgrade redo log format, not backward compatible
5. Change redo log type IDs
6. Code cleanup
7. Shard the fil system data structures for performance
8. Remove WL#9499 files that were used to track open tablespaces.

RB#16842 Approved by: Jimmy and Kevin.