Use AuthSession cookie for login instead of basic auth

[dianabarsan]

Describe the issue
We're discussing increasing pbkdf2 iterations for CouchDb.
This will mean that computing the hashed password every time basic auth is used will become a costly server-side operation.

Describe the improvement you'd like
Instead of using basic auth for every request, make one initial _session request, and use the provided AuthSession cookie to authenticate all following requests within one conf run.

Describe alternatives you've considered
None.