Merge #17728: rpc: require second argument only for scantxoutset start action

7d263571bee8c36fbe3c854b69c6f31cf1ee3b9b rpc: require second argument only for scantxoutset start action (Andrew Chow)

Pull request description:

  It was reported on [IRC](http://www.erisian.com.au/bitcoin-core-dev/log-2019-12-11.html#l-377) that `scantxoutset`'s API was broken in 0.19.0:

  ```
  <belcher> i think scantxoutset may have been broken in bitcoin core 0.19 ? regardless of what parameters i run it with (e.g. "scantxoutset abort", "scantxoutset status") it just returns the help doc, according to the release notes the only change was https://github.com/bitcoin/bitcoin/pull/16285/files but i dont see anything that wouldve broken it, it works fine in 0.18
  <belcher> im on regtest, in case its important
  <harding> I can confirm `scantxoutset abort` returns the help doc on latest master.  Waiting for 0.18.1 to start now to attempt to reproduce there.
  <harding> It looks like it's expecting a second parameter (even though that doesn't make sense with "abort").
  <jonatack> Same for me as well
  <harding> Can also confirm that `scantxoutset abort` returns the expected result on 0.18.1.
  ```

  As noted in the conversation, previously, the second argument of `scanobjects` is only required for the `start` action. `Stop` and `abort` actions did not and could work without them.

  It appears that this was broken by #16240 which enforced the size of the arguments to match the listed required arguments.

  To fix this issue, this PR makes the `scanobjects` argument an optional argument. Then only in the `start` action do we check whether the `scanobjects` argument is there and throw an informative error about that. Also a test is added for this case.

ACKs for top commit:
  laanwj:
    ACK 7d263571bee8c36fbe3c854b69c6f31cf1ee3b9b
  promag:
    ACK 7d263571bee8c36fbe3c854b69c6f31cf1ee3b9b.

Tree-SHA512: 828bdfe47f4fffa5d00a2cf88db6cea4a2714d9c49276841ca5cbdd1603b87bb6862147b86edcf36d7b40314ddb80b1a07fd399faf288572c55cc788c5cf9526

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