Roll up addColumn and dropColumn changes for MySQL (CORE-1833)

See https://liquibase.jira.com/browse/CORE-1833 for details.

Currently the produced SQL for MySQL for the addColumn change is inefficient, if multiple columns are added at once. In that case, only one ALTER TABLE statement should be produced which can add the new columns in one go.
A similar approach should be possible with dropColumn.

This implementation changes now AddColumnStatement and DropColumnStatement to maintain a list of columns to be added/dropped. No AlterTableStatement is introduced. AddColumnChange and DropColumnChange generate these "multiple column aware" statements. 

See #260 and #311 for the old pull requests.