WL#2708: Speed up ALTER TABLE (part 7: Online ALTER TABLE, MyISAM)

See WL#2289 for task background.

Overall description:

- Online ALTER TABLE for MyISAM
  This will allow one to do reads, inserts and updates (not drop/rename)
  to the table while the ALTER TABLE is running. All changes will
  be applied to the copy of the table before the newly created
  ALTER TABLE replaces the old one.
  This will allow on to do any changes to the table except:
  - Rename table
  (Note that adding a new UNIQUE INDEX may be problematic, but not
  impossible)