webhttp storage can get json documents by calling http GET method. for jio allDocs,
the storage will get a file called _document_list which should contain the list
of document id that can be downloaded.

replicatedopmltree storage can convert a list of opml storage (each opml can contain
one or more sub_storage) into an indexeddb. All document in opml tree will have an
entry into the indexeddb storage, the full document is added as attachment in the
indexeddb.