KeyValueAccumulator performance improvements

[davidfowl]

• It allocates 2 dictionaries
• It should probably be a struct

[davidfowl]

Even better would be to make the KeyValueAccumulator a call back model so there was 0 allocation (besides the delegate 😄 )

[muratg]

If you have any proof of large perf issues with this, bring it back

[benaadams]

@muratg this was completed in #411 (comment)