fix duplicate results in get_key_references

[oxarbitrage]

Pull for #1209

The issue is also part of #1753

I think we should work in the dupes here while leaving #1753 for pagination.

This pull request implements flat_set as described by @abitmore here #1753 (comment) to solve the issue.

[abitmore]

I think it's better to create a special test case for reproducing and fixing the duplicates.

[oxarbitrage]

Ill add, i tested in a live chain and it worked(duplicates were gone), but agree test cases will be the best.

[abitmore]

Hmm. It's a simple change, perhaps writing a test case is not worth the efforts.

[oxarbitrage]

Yes, it is not that easy to create a test case for this. If we can leave that we save some time unless it is totally necessary.

By the way here is a sample in the cli_wallet of before:

new >>> get_key_references [BTS5eTPR9wSnWSiTtJNhqheXbjAmvacq5tycn1RU7eixhSuszidMY] 
[[
    "1.2.93139",
    "1.2.93139"
  ]
]

And after:

new >>> get_key_references [BTS5eTPR9wSnWSiTtJNhqheXbjAmvacq5tycn1RU7eixhSuszidMY] 
[[
    "1.2.93139"
  ]
]
new >>>

The API call after fix:

oxarbitrage@oxarbitrage-Lenovo-ideapad-320S-14IKB:~$ curl --silent -d '{"id":1,"method":"call","params":["database","get_key_references",[["BTS5eTPR9wSnWSiTtJNhqheXbjAmvacq5tycn1RU7eixhSuszidMY"]]]}' http://127.0.0.1:8090/ws | jq
{
  "id": 1,
  "jsonrpc": "2.0",
  "result": [
    [
      "1.2.93139"
    ]
  ]
}
oxarbitrage@oxarbitrage-Lenovo-ideapad-320S-14IKB:~$