[DBAL-234] Add support for renaming indexes

Currently renamed only indexes are scheduled for removal and recreation in the comparator. However recreating an index when only the name has changed is very inefficient on large tables as it internally requires a table recreation on most of the platforms. Many vendors offer a more efficient syntax for renaming indexes which saves the table recreation. This PR introduces this behaviour on capable platforms. Platforms that do not support this kind of syntax still use the current behaviour of dropping and recreating an index.
This PR also introduces the `Table::renameIndex()` method and the `MySQL57Platform` platform which adds [support for explicitly renaming indexes](http://dev.mysql.com/doc/refman/5.7/en/mysql-nutshell.html).