Singular way to signal iteration end

From https://github.com/Level/concat-iterator/issues/1#issuecomment-399747452:

So atm, there’s two ways in the ecosystem to detect the end of iteration:

1. arguments.length === 0
2. key === undefined && value === undefined

We gotta pick one and stick with it.

And

this should be specified in abstract-leveldown and the tests should enforce this.

[vweevers]

In other words, no change is required in abstract-leveldown, but since we aren’t supporting undefined anyway, we should make that official.

[vweevers]

I prefer the key-value check, it covers more ground. Because if no arguments are passed, then key === undefined && value === undefined is also true.

@ralphtheninja WDYT?