Truncate table (Empty table form all data)

MySQL does truncate by doing a create of an existing table (if table handler
supports recreate). In some circumstanes MySQL does call a handler function
'delete_all_rows' to delete all rows, but still remember things like index
statics and the last auto_increment value.

Maria will initially support recreate of table and later, if needed,

maria_delete_all_rows()



maria_delete_all_rows() can be rollbacked, TRUNCATE TABLE can't.

 TRUNCATE is done as a normal CREATE (WL#3239) but with a flag in the

log that any old files should be overwritten.



The suggested way to implement maria_delete_all_rows is as follows:



- Take a Maria table lock

- Loop over all rows and mark them deleted

- Do same with all index entries

- Instead of creating undo-purge entry's for all data, create one

 'undo delete_all_rows' entry.

- Drop Maria table lock (versioning will handle conflicts)



- In case of commit:

  Use delete-update handler to scan through all data  and index for the

  table and delete all entries marked delete by this transaction.

- In case of rollback:

  Scan through all data  and index for the table and remove delete marker

  for all entries marked delete by this transaction.

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