WebSQL : When dropping a store, also drop multi-entry indexes.
WebSQL : When dropping a store, also drop multi-entry indexes.

In Websql implementation, the library create a separated table for each
multi-entry index in a store. On store changes, or store removal, the
library must also drop the table corresponding to those indexes.

yathit/ydn-db,yathit/ydn-db,yathit/ydn-db,yathit/ydn-db,yathit/ydn-db