Description: This PR aims to mergeSqlFileStream from the netfx and netcore projects. The strategy for this merge is to use the Interop.* libraries from the netcore project to enable merging the classes. Therefore, the code for this class is mostly based on the netcore implementation with a handful of#if NETFRAMEWORK blocks to add overrides for netfx behavior.

The commits are broken up into fairly digestible chunks that explain each step along the process. I'd recommend reviewing the PR by stepping through the commits and seeing if you agree with the changes. Nevertheless, this PR has a lot of code cleanup changes at the end that make the overall PR fairly large. In anticipation of pushback on this, I will offer a separate PR that only contains the merge changes. If that one is accepted, I'll still send out the cleanup change on a separate PR. But, I think this option would be faster overall.

Testing: The existing manual tests for SqlFileStream were used to validate behavior.