Replace unbounded channels with bounded ones. (#1191)

* Replace unbounded channels with bounded ones.

To remove the unbounded channels used for communicating with node tasks
an API similar to `futures::Sink` is used, i.e. sending is split into a
start and complete phase. The start phase returns `StartSend` and first
attempts to complete any pending send operations. Completing the send
means polling until `Poll::Ready(())` is returned.

In addition this PR has split the `handled_node_tasks` module into
several smaller ones (cf. `nodes::tasks`) and renamed some types:

- `nodes::handled_node_tasks::NodeTask` -> `nodes::tasks::task::Task`
- `nodes::handled_node_tasks::NodeTaskInner` -> `nodes::tasks::task::State`
- `nodes::handled_node_tasks::NodeTasks` -> `nodes::tasks::Manager`
- `nodes::handled_node_tasks::TaskClosedEvent` -> `nodes::tasks::Error`
- `nodes::handled_node_tasks::HandledNodesEvent` -> `nodes::tasks::Event`
- `nodes::handled_node_tasks::Task` -> `nodes::tasks::TaskEntry`
- `nodes::handled_node_tasks::ExtToInMessage` -> `nodes::tasks::task::ToTaskMessage`
- `nodes::handled_node_tasks::InToExtMessage` -> `nodes::tasks::task::FromTaskMessage`

* `take_over_to_complete` can be an `Option`.

Since it is always holding just a single pending message.

* `send_event_to_complete` can be an `Option`.

* Update core/src/nodes/tasks/manager.rs

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

* Update core/src/nodes/tasks/manager.rs

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

* Add comments to explain the need to flush sends ...

of take-over and event messages delivered over Sinks.

swarm/src/protocols_handler/node_handler.rs

@@ -30,8 +30,7 @@ use libp2p_core::{
     ConnectedPoint,
     PeerId,
     nodes::collection::ConnectionInfo,
-    nodes::handled_node::{NodeHandler, NodeHandlerEndpoint, NodeHandlerEvent},
-    nodes::handled_node_tasks::IntoNodeHandler,
+    nodes::handled_node::{IntoNodeHandler, NodeHandler, NodeHandlerEndpoint, NodeHandlerEvent},
     upgrade::{self, InboundUpgradeApply, OutboundUpgradeApply}
 };
 use std::{error, fmt, time::Duration};