Memory mapped files for communication with taskhosts #13561

Based on #13372 we see there is a large overhead when communicating with out of process Taskhosts (any IPC really, but there we were considering tasks on taskhost).

There was an exploration of using memory mapped files to remove some of this overhead here with promising numbers #11502

Decreasing overhead improves the adoption story for #4834 and -mt (where taskhosts are used extensively as fallback mechanism for nonmigrated tasks).