Incrementally send RPC data.

A reprise of https://github.com/ripple/rippled/pull/696
I fail to see the benefit of this type over `std::function<void> yield` in argument lists. But I see a disadvantage. Anyone who wants their declaration to include a value of type `Yield` now has to see all the declarations introduced by `<boost/coroutine/all.hpp>`. Even though they aren't using it.