Compare-and-swap (CAS) command as a building block for synchronization primitives

[elifarley]

From Wikipedia article on CAS:

This operation is used to implement synchronization primitives like semaphores and mutexes, as well as more sophisticated lock-free and wait-free algorithms.

CAS will be useful in #5144 for instance.

Proposed syntax:

// When you want to use CAS for a few attributes only:
r.table("posts").get(1).update({
    title: r.compareAndSwap("Lorem ipsum OBSOLETUS", "Lorem ipsum NOVUS"),
    content: "Dolor sit amet"
})

// When you want to use CAS for many attributes:
r.table("posts").get(1).update( r.compareAndSwap(
{
    title: "Lorem ipsum OBSOLETUS",
    content: "Dolor sit amet OBSOLETUS"
}, {
    title: "Lorem ipsum NOVUS",
    content: "Dolor sit amet NOVUS"
}
))

This could be internally converted to something like

r.table("posts").get(1).update({
    title: r.branch(
      r.row('title').eq("Lorem ipsum OBSOLETUS"),
      "Lorem ipsum NOVUS",
      r.error(r.row('title')) // return an error indicating what was the unexpected value
    )
})