[ws client]: register request IDs for unsubscribe requests. (#223)

* [ws client]: register request ID for unsubscribe.

Fixes #220

* refactor manager usage

* restrict request ID to u8

* Update ws-client/src/tests.rs

* rename `max_notifs_per_subscription_capacity`

* Update tests/src/lib.rs

* fix nits

* address grumbles: vecdeq for free slots

* resolve todo

* [request manager]: add test for max limit.

* Update ws-client/src/client.rs

Co-authored-by: David <[email protected]>

* fix stack overflow

* grumbles: MaxMemory -> MaxSlots

* address grumbles: stray capacity on configs

* Update types/src/client.rs

Co-authored-by: David <[email protected]>

* grumbles: stray confusing docs

* grumbles: doc nits

* grumbles: remove nested if let

Co-authored-by: David <[email protected]>