New methods to drop or rename extension index in DatabaseUpdater

- DatabaseUpdater::dropExtensionIndex( $tableName, $indexName, $sqlPath )
- DatabaseUpdater::renameExtensionIndex( $tableName, $oldIndexName, $newIndexName, $sqlPath, $skipBothIndexExistWarning = false )

[ Case use ]

This feature is required to help extensions schema change to ensure
compatibility with SQLite.

First extensions to use it will be Echo and Flagged Revisions.

Change-Id: Ia2782d644593ab6b64b67720ed61b3994db10346