Add support for online table alters

[Shelnutt2]

The idea is to add support for online table alters by creating new version of the schema. We can then produce a migration of data on read. This would put the work on reads instead of causing a table rebuild when a column is changed. During file consolidation older data files can be upgraded to the latest version of the schema.

If column order is what is being changed nothing has to be done. Even if ondisk storage and capnp proto schema is in different order, the fields are set using the field interface, so no changes are needed.

If column name changes, then we can just create a new capn proto schema with the new names, but as long as order and data types are the same, then the data on disk doesn't need to change.

Changes are needed for:

• ADD_COLUMN
• ALTER_COLUMN_DEFAULT
• ALTER_COLUMN_NULLABLE
• ALTER_COLUMN_FORMAT
• ALTER_COLUMN_STORAGE_TYPE

Implement:

• check_if_supported_inplace_alter (TABLE *altered_table, Alter_inplace_info *ha_alter_info)
• prepare_inplace_alter_table(TABLE *altered_table, Alter_inplace_info *ha_alter_info);
• inplace_alter_table(TABLE *altered_table, Alter_inplace_info *ha_alter_info)
• commit_inplace_alter_table(TABLE *altered_table, Alter_inplace_info *ha_alter_info, bool commit);
• notify_table_changed();

[Shelnutt2]

Right now the only miss component is default values need to be added to the schema so when messages are updated, and new columns have no values, we don't have to take up space by adding it, we should just use capnp default value support.