chainHead_storage: Iterate over keys (#14628)

* chainHead: Iterate over key,values and key,hashes

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

* chainHead/tests: Multi query with iteration over keys

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

* chainHead/events: Fix typo in StorageQuery

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

* chainHead: Take 10 from key iterator

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 <>