Use non-retaining command buffers on Metal

[kvark]

From https://developer.apple.com/documentation/metal/mtlcommandqueue/1508684-makecommandbufferwithunretainedr :

Use this method only in extremely performance-critical applications where the cost of maintaining references to objects is significant, and references by some other part of the application can be guaranteed until the object is no longer needed for command execution.

So one important point is Apple did go this extra step to provide a method, meaning that the performance difference is expected. Since we are manually tracking the lifetimes on other backends, we should be using this code paths on Metal, minus some cases where we currently rely on tracking:

• buffer updates of Metal update buffer #1777