feat: add a way to limit the number of subscriptions per connection (#739)

* feat: limit the number of subscriptions

Closing #729

* fix nit

* Update core/src/server/helpers.rs

* add integration tests + some fixes so it works

* cargo fmt

* fix doc links

* Unsubscribe calls should avoid subscription limits

Point to Tokio 1.16 (we use a method from it), and a little special treatment for unsubscribe methods

* No resource limiting for Unsubscribe calls

* Test that we can still unsubscribe after hitting a limit

* Fix a comment typo

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

* Update core/src/server/rpc_module.rs

* Update core/src/server/rpc_module.rs

Co-authored-by: James Wilson <james@jsdw.me>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
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