RDS-1741 - Implement some missing checks.

[bartossh]

IMPLEMENTED ALREADY:

• check if all timestamps of the processed payload are equal and non-zero ( in Payload.get_min_validated_timestamp() )

• check if the data-point value-size is not exceeding 32 bytes (trait sanitized - panics if value exceeded)

• check the vulnerability for the ecdsa signature malleability issue

DONE:

• check if the signer list doesn’t contain duplicates

• check if the feed_id list doesn’t contain duplicates

• check if the feed_id list is non-empty

• check the vulnerability for the ecdsa signature malleability issue

[kostekIV]

Nice code, love the tests and docs. Left some nitpicks