Simplify handling of parallelism

Default parallelism to IdealThreadCount+1, remove checks for input parallelism
> IdealThreadCount, since we find that a higher thread count can help in some
circumstances.

Don't use release/reserve thread to increase the size of the thread pool in
init, instead just set the max pool size directly.
Add comments on currently disabled transforms

Also, remove some extraneous curly braces.