It was reported on IRC 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.
