Roll up addColumn and dropColumn changes for MySQL

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.

Note: This improvement is similar to https://liquibase.jira.com/browse/CORE-46 but only for one single change.