IDB insertions performance

[leplatrem]

Iterating on list in order to perform massive IndexedDB insertions is not efficient.

We should chain insertions instead for the browser to schedule the insertions better.

[leplatrem]

For the bulk insert in Collection.importChanges, maybe we should iterate in chunks instead. Modifying the storage adapter interface is less obvious. But maybe we could rely on loadDump() there too :) I'll investigate