Revert "Disable buffer recycling in FileStream which was causing problems when pinning buffers for interop. Also I don't believe this optimization is needed with sgen now."

This reverts commit 255a46e8eb8a05a495b80a3e5d0b5acd2ddf425e.
Revert "Restore finalizer and GC.SuppressFinalize."

This reverts commit 40ed4cf1b48fd596f585c154934a0a6cc6a5eacc.
Enable xdebug on osx.