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.

core/src/nodes/tasks/mod.rs

@@ -0,0 +1,45 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+//! Management of tasks handling nodes.
+//!
+//! The core type is a [`task::Task`], which implements [`futures::Future`]
+//! and connects and handles a node. A task receives and sends messages
+//! ([`tasks::FromTaskMessage`], [`tasks::ToTaskMessage`]) to the outside.
+//!
+//! A set of tasks is managed by a [`Manager`] which creates tasks when a
+//! node should be connected to (cf. [`Manager::add_reach_attempt`]) or
+//! an existing connection to a node should be driven forward (cf.
+//! [`Manager::add_connection`]). Tasks can be referred to by [`TaskId`]
+//! and messages can be sent to individual tasks or all (cf.
+//! [`Manager::start_broadcast`]). Messages produces by tasks can be
+//! retrieved by polling the manager (cf. [`Manager::poll`]).
+
+mod error;
+mod manager;
+mod task;
+
+pub use error::Error;
+pub use manager::{ClosedTask, TaskEntry, Manager, Event, StartTakeOver};
+
+/// Task identifier.
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord)]
+pub struct TaskId(usize);
+