chainHead: Error on duplicate unpin hashes (#3313)

This PR addresses an issue where calling chainHead_unpin with duplicate
hashes could lead to unintended side effects.

This backports:
https://github.com/paritytech/json-rpc-interface-spec/pull/135



While at it, have added a test to check that the global reference count
is decremented only once on unpin.

cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: Davide Galassi <[email protected]>