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outdated.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.
