Add protocols_handler::multi module. (#1497)

* Add `protocols_handler::multi` module.

An implementation of `ProtocolsHandler` that contains multiple other
`ProtocolsHandler`s indexed by some key type.

* Randomise start position of handler polling.

* Address some review concerns.

* Add `IntoMultiHandler`.

* Check protocol names for uniqueness.

* Changes due to review.

- No more `Debug` bound for the key type and more generic log messages.
- Additional comments.
- Imports instead of fully-qualified use.
- Renamed `DuplicateProtoname` to `DuplicateProtonameError`.

* Replace `HashMap` with `Vec` in `Upgrades`.

* Review suggestion.

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>
Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

swarm/src/protocols_handler.rs

@@ -43,6 +43,7 @@ mod map_out;
 mod node_handler;
 mod one_shot;
 mod select;
+pub mod multi;
 
 pub use crate::upgrade::{
     InboundUpgradeSend,