* client/*: Treat protocol name as str and not [u8]

Notification protocol names are in practice always valid utf8 strings.
Instead of treating them as such in the type system, thus far they were
casted to a [u8] at creation time.

With this commit protocol names are instead treated as valid utf8
strings throughout the codebase and passed as `Cow<'static, str>`
instead of `Cow<'static, [u8]>`. Among other things this eliminates the
need for string casting when logging.

* client/network: Don't allocate when protocol name is borrowed

* Don't close inbound notifications substreams immediately

* Fix not closing in return to node A closing

* Add NetworkService::send_notifications

* Doc

* Doc

* API adjustment

* Address concerns

* Make it compile

* Start implementation

* Progress in the implementation

* Change implementation strategy again

* More work before weekend

* Finish changes

* Minor doc fix

* Revert some minor changes

* Apply suggestions from code review

* GroupError -> NotifsHandlerError

* Apply suggestions from code review

Co-authored-by: Roman Borschel <[email protected]>

* state_transition_waker -> close_waker

* Apply suggestions from code review

Co-authored-by: Roman Borschel <[email protected]>

* Finish renames in service.rs

* More renames

* More review suggestsions applied

* More review addressing

* Final change

* 512 -> 2048

Co-authored-by: Roman Borschel <[email protected]>

* Fix lots of small nits in sc-network

* Update client/network/src/protocol/sync/blocks.rs

Co-authored-by: Arkadiy Paronyan <[email protected]>

* Fix warning

* Yes. The line width.

Co-authored-by: Arkadiy Paronyan <[email protected]>

* Temporarily increase notifications buffer size

* Add a 511.0 bucket

* Better logging for notifications and buffer size increase

* Address review

* Improve warning about notifications queue and remove spurious triggers

* Add a sub_libp2p_notifications_queues_size Prometheus metric

* Fix network tests

* Address review

* Fix tried to send handshake twice

* Fix wrong boolean

* Change to debug

If polling encounters a `Poll::Pending` we need to return it instead of
continuing the loop which may turn it into a blocking operation, causing
problems with executors.

* Open one substream for each notifications protocol

* Fix WASM build

* Apply suggestions from code review

Co-Authored-By: Toralf Wittner <[email protected]>

* Address concerns

* Use unsigned-varint to read the varint

* Use unsigned-varint

* Forgot Cargo.lock

Co-authored-by: Toralf Wittner <[email protected]>