use transform-pouch instead of pouchdb-wrappers

[gr2m]

pouchdb-users hooks into db.put(), db.post() and db.bulkDocs() in order to transform a user doc into the format that CouchDB uses for its _users database. For example, it removes the .password proprety and replaces it with a .derived_key.

We currently use pouchdb-wrappers for that, although transform-pouch is a much better supported and tested library that does the same thing

Bonus (we can make a follow up issue for that): remove the Promise and make db.installUsersBehavior a sync operiation

[raymonddeng99]

Can you delegate this to me? I can add transform-pouch to the index.js file.

[gr2m]

Sure, it's all yours, thanks for looking into it!