Condition choices may need to hit the database or perform some other

expensive computation in order to make a match determination. Sometimes

these conditions need the same information, or sometimes a condition

will be used more than once. In these cases, it can be handy for the

implementations of those choices to be able to cache some common state,

like the results of the database queries.
This adds a new, optional value_state_cache keyword argument to the

get_match_value() methods, which can be used to populate that cache. The

cache is local to a ConditionSet match run, and does not otherwise

persist.
Along with this, BaseConditionChoice.get_match_value() and

BaseConditionOperator.matches() now take a **kwargs, in order to allow

for future expansion (and to swallow the new value_state_cache argument

for methods that don't need to use it).

Unit tests pass.
Made use of the new caching in an implementation.