chainHead: Produce method responses on `chainHead_follow` (#14692)

* chainHead/api: Make storage/body/call pure RPC methods

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

* chainHead: Add mpsc channel between RPC methods

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

* chainHead/subscriptions: Extract mpsc::Sender via BlockGuard

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

* chainHead/subscriptions: Generate and provide the method operation ID

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

* chainHead: Generate `chainHead_body` response

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

* chainHead: Generate `chainHead_call` response

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

* chainHead: Generate `chainHead_storage` responses

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

* chainHead: Propagate responses of methods to chainHead_follow

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

* chainHead/tests: Adjust `chainHead_body` responses

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

* chainHead/tests: Adjust `chainHead_call` responses

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

* chainHead/tests: Adjust `chainHead_call` responses

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

* chainHead/tests: Ensure unique operation IDs across methods

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

* chainHead/events: Remove old method events

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

* chainHead: Return `InvalidBlock` error if pinning fails

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

* chainHead: Wrap subscription IDs

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

* chainHead/tests: Ensure separate operation IDs across subscriptions

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

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