LevelDB: Support AUTO_INCREMENT

For start, support AUTO_INCREMENT in a very basic way:

Maintain next_free_value number

• When opening the table, load it from the index
• Increase it whenever anybody reserves AUTO_INCREMENT values (regardless of whether they use them or not. InnoDB doesn't guarantee continutity, either)
• Also increase when writing a record: if (new_record.pk > next_free_value) { next_free_value= new_record.pk+1; }

next_free_value can be stored in TABLE_SHARE.

MyISAM supports auto_increment for columns that are not at key start. We will not support this.

InnoDB does some tricks related to binary logging. We will not do this.