* Add Playground struct and basic support for creating it and retrieving
it
* Add missing functions in pwd mock
* Get playground from request domain and validate it exists. If valid set
it on the newly created session.
* Move playground specific configurations to the playground struct and use
it everytime we need that conf.
* Don't allow to specify a duration bigger that the allowed in the
playground
Remove global `sessions` map and use configured storage.
Add a `types` package so both `pwd` and `storage` can access without
circular dependencies.
Now the session is prepared when requested and not on load.
