swarm/: Make NodeHandlerWrapper an impl detail of connection (#2523)

Previously one would wrap a `ProtocolsHandler` into a
`NodeHandlerWrapper` as early as possible, even though the functionality
of `NodeHandlerWrapper` is only needed within `mod connection`.

This commit makes `NodeHandlerWrapper` an implementation detail of `mod
connection`, thus neither `mod protocols_handler`, `mod pool` nor the
root level (`libp2p-swarm`) need to bother about the abstraction.

In addition to the above, this commit:

- Renames `NodeHandlerWrapper` to `HandlerWrapper`. The word `Node` is
  outdated.
- Removes `NodeHandlerWrapperBuilder`. With this simplification it is no
  longer needed.
- Folds `NodeHandlerWrapperError` into `ConnectionError`. No need for
  upper layers to be aware of the fact that `ProtocolHandler`s are
  wrapped.

swarm/src/protocols_handler.rs

@@ -42,7 +42,6 @@ pub mod either;
 mod map_in;
 mod map_out;
 pub mod multi;
-mod node_handler;
 mod one_shot;
 mod select;
 
@@ -55,10 +54,6 @@ use std::{cmp::Ordering, error, fmt, task::Context, task::Poll, time::Duration};
 pub use dummy::DummyProtocolsHandler;
 pub use map_in::MapInEvent;
 pub use map_out::MapOutEvent;
-pub use node_handler::{
-    NodeHandlerWrapper, NodeHandlerWrapperBuilder, NodeHandlerWrapperError,
-    NodeHandlerWrapperEvent, NodeHandlerWrapperOutboundOpenInfo,
-};
 pub use one_shot::{OneShotHandler, OneShotHandlerConfig};
 pub use select::{IntoProtocolsHandlerSelect, ProtocolsHandlerSelect};
 
@@ -222,17 +217,6 @@ pub trait ProtocolsHandler: Send + 'static {
     {
         ProtocolsHandlerSelect::new(self, other)
     }
-
-    /// Creates a builder that allows creating a `NodeHandler` that handles this protocol
-    /// exclusively.
-    ///
-    /// > **Note**: This method should not be redefined in a custom `ProtocolsHandler`.
-    fn into_node_handler_builder(self) -> NodeHandlerWrapperBuilder<Self>
-    where
-        Self: Sized,
-    {
-        IntoProtocolsHandler::into_node_handler_builder(self)
-    }
 }
 
 /// Configuration of inbound or outbound substream protocol(s)
@@ -493,15 +477,6 @@ pub trait IntoProtocolsHandler: Send + 'static {
     {
         IntoProtocolsHandlerSelect::new(self, other)
     }
-
-    /// Creates a builder that will allow creating a `NodeHandler` that handles this protocol
-    /// exclusively.
-    fn into_node_handler_builder(self) -> NodeHandlerWrapperBuilder<Self>
-    where
-        Self: Sized,
-    {
-        NodeHandlerWrapperBuilder::new(self)
-    }
 }
 
 impl<T> IntoProtocolsHandler for T