Avoid rebuilding combinator reset delegate

[glopesdev]

Operator properties can sometimes represent shared state which is meant to be assigned and used only at runtime, and which needs to be explicitly invalidated in between workflow execution (see #1083).

Such operators can use the ResetCombinatorAttribute to indicate they want any non-serializable properties (i.e. properties marked with the XmlIgnoreAttribute) to be reset to their default values on every build. Currently this is implemented as a delegate which is emitted at build time for every resettable operator. However, since the delegate logic depends only on the combinator type, it would be possible to cache the emitted delegates to avoid re-emitting delegates for multiple uses of the same operator.

This PR introduces a nested generic type to serve as the cache, which should speed up build times in workflows with large numbers of resettable operators.