Before this pr we had the `TaskExecutor` abstraction which theoretically
allowed that any futures executor could have been used. However, this
was never tested and is currently not really required. Anyone running a
node currently only used tokio and nothing else (because this was hard
coded in CLI). So, this pr removes the `TaskExecutor` abstraction and
relies directly on the tokio runtime handle.

Besides this changes, this pr also makes sure that the http and ws rpc
server use the same tokio runtime. This fixes a panic that occurred when
you drop the rpc servers inside an async function (tokio doesn't like
that a tokio runtime is dropped in the async context of another tokio
runtime).

As we don't use any custom runtime in the http rpc server anymore, this
pr also removes the `rpc-http-threads` cli argument. If external parties
complain that there aren't enough threads for the rpc server, we could
bring support for increasing the thread count of the tokio runtime.