Allow for customization of the size of the CQL expression cache in the engine

[acoteathn]

My team uses the Java CQL engine to evaluate our CQL libraries. We've had great runtime performance with the majority of our CQL, but recently we added a new library that has been performing worse and we're struggling a little to understand why.

One thing I'm suspect of is the hard-coded limit of 15 expressions in the expression cache: https://github.com/cqframework/clinical_quality_language/blob/master/Src/java/engine/src/main/java/org/opencds/cqf/cql/engine/execution/Cache.java#L36

As well as the hard-coded limit of 10 libraries in cache at any given time: https://github.com/cqframework/clinical_quality_language/blob/master/Src/java/engine/src/main/java/org/opencds/cqf/cql/engine/execution/Cache.java#L27

Would it be possible to expose those values as configuration overrides we could specify when constructing the CqlEngine?