Allow to create view with limited threads count(Experimental dispatcher).

[e5l]

Consider the following use case:

val client = HttpClient(threads = 2)
val server = HttpServer(threads = 6)

class HttpClient(threads: Int) {
    val dispatcher = DefaultDispatcher.limited(threads)
}
...

[elizarov]

This is related to #261. We need to provide ability to create thread-limited blocking and non-blocking dispatchers. It does not see that the corresponding function need to be prefixed with DefaultDispatcher. Since it is a default, some top-level functions shoud do just fine.

[qwwdfsad]

I've introduced a corresponding API for the experimental dispatcher.
Top-level functions (or members of DefaultDispatcher) will be introduced along with #261