Add resource limiting for `Subscriptions` (#786)

* ws-server: Fix copyright for tests

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* rpc_module: Return a resource builder when subscribing

Registering a subscription returns the subscription'
callback wrapped into a `MethodResourcesBuilder` for resource
limiting purposes.

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Fix `register_subscription` tests

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* server: Drop `ResourceGuard` with `SubscriptionSink` for resource limit

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Check resource limits for subscription

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* proc-macros: Render resource limits for subscription macro

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Extend subscription limiting test via macro generation

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* core: Check if the `unsubscribe` method was already inserted

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Fix unsupported fields for subscriptions

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* server: Verify subscription methods before registering them

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update test comment for subscription limiting

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Modify tests comments

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>