Update pouchdb-adapter-http to support session authentication instead of basic authentication

[dianabarsan]

Issue

Best practices for hashing passwords is to use the highest number of iterations possible. In CouchDb, the recommended number of iterations is 10000, with this disclaimer:

A higher number provides better hash durability, but comes at a cost in performance for each request that requires authentication. When using hundreds of thousands of iterations, use session cookies, or the performance hit will be huge.
Source

pouchdb-adapter-http uses basic authentication exclusively.

I'm proposing to update pouchdb-adapter-http so that it supports using session authentication, along with basic authentication.

[dianabarsan]

I've already developed a plugin that wraps pouchdb-adapter-http to add a call to _session and/or append the resulting AuthSession cookie header for outgoing requests: https://github.com/medic/pouchdb-session-authentication

I'm keen to work to include similar logic into the adapter directly.