[www] Make www scalable

[marcopeereboom]

Politeia www layer currently uses a leveldb database for user data. In order to make www scale that database needs to become distributed. In addition we need to move some values that we currently store on disk also in the new database layer (session keys, cookies, and some other secret material). These values are sensitive so they must be stored encrypted.

• Pick scalable database (RoachDB, Reddis, Mongo, ?)
• Add a new database to www user interface.
• Create migration tools to move users to the new database.
• Determine how to move secret bits into the database (or if we want some sort of agent that provides secrets in a different way)
• Create migration tools to move secrets to the new database/agent.

[lukebp]

I think any of those database options would be fine for the user database, with marginal benefits over one another. It may be best to use the same database that we choose for #546 in order to keep complexity to a minimum.

Side note: the contractor management system has swapped out leveldb for CockroachDB.

[marcopeereboom]

We agree on that. Whatever #546 determines will become the same target. I think at this point that we have determined cockraochdb is the way to go and it is fortunately compatible with postgres.

So what we will need is to migrate all on disk bits that currently are secret into the database but encrypted.

The gorilla session package can have multiple backends. We should probably fork https://github.com/antonlindstrom/pgstore and add the encryption bits to that. Similarly we need to modify the api to encrypt stuff on the fly that is secret.

When everything is said and done www should ONLY have a configuration file and logs stored on disk. We also should debate offloading logs to some sort of repo but that is a different issue/pr.

[marcopeereboom]

This has been achieved with various commits.