chainHead/storage: Fix storage iteration using the query key (#1665)

This PR ensures that all storage keys under a prefix are returned by the
`chainHead_storage` method.

Before this PR, the `storage_keys` was used with just the `start_key`.
Before the pagination event was generated, the last reported key
`last_key` was saved internally.
When the pagination is resumed, the `last_key` will serve as the next
`start_key` to the `storage_keys` API.

However, this behavior does not function properly for non-prefixed
storage keys.

Entry keys `a`, `ab`, `abc` share a common prefix and therefore the `ab`
key leads to `abc`.
However, for `a`, `ab`, `aB` and `abc`, the `aB` key does not
immediately lead to `abc`.

To mitigate this, the PR saves the start key of the query, together with
the next pagination key.
Improve testing to ensure we have a key entry that doesn't share the
prefix with the descendant key.

@paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <[email protected]>