API for caching attribute values

[fluffyemily]

Allow the caching of attribute data.

Create a mechanism by which a client can pass a keyword into mentat and allow that values against that keyword to be cached in memory.

Things to think about:

• Do we want 2 levels of caching, eager and lazy? Or just one, in which case do we want either eager or lazy caching?
• Do we want a cache per connection? If so, where should this go and how do we ensure that we don't have multiple copies of the same cache? If not, what is the best way of sharing that cache between threads?