Improving the maintainability of the code

[asppsa]

We currently have a B rating on Code Climate, largely due to some adapters getting very bad ratings - see https://codeclimate.com/github/moneta-rb/moneta/code?sort=maintainability

The worst offenders are:

• adapters/sequel.rb (F)
• adapters/activerecord.rb (D)
• adapters/couch.rb (D)
• adapters/cassandra.rb (C)

Oh dear - these are all files that I have worked on!

The transformer proxy is also getting a poor score, but we should look at a broader refactor of that - see #176.