Adjust behavior of conflated channel to match use-case of intermediate values

[elizarov]

See long discussion in #332. Here is TL;DR. Current behavior of Channel(CONFLATED) (ConflatedChannel) can be demonstrated with the following code (play):

fun main() = runBlocking {
    val channel = produce(capacity = Channel.CONFLATED) { 
        send(1); send(2); send(3); send(4)
    }
    channel.consumeEach { println(it) } // prints only 1
}

What happens here is that closing a channel is considered to send a "value" that overwrites last sent value. This is subtle behavior and it is not clearly spelled out in docs. In fact, on can argue that docs imply a different behavior. Anyway, this is not what one wants when conflated channel is used to send intermediate values of some computation. In this case one needs to ensure that the most recent value is always delivered. This use-cases seems to be most prominent one and this proposal is to change behavior of conflated channels to match it, so that the above code prints 1 and 4.