[0.17] Bugfix: Correctly calculate balances when min_conf is used, and for getbalance("*")

[luke-jr]

0.17 backport of #14602, but with named-arg account="*" compatibility

[meshcollider]

Concept ACK

[gmaxwell]

Concept ACK.

[jonasschnelli]

utACK 32de31c

[jnewbery]

I have the same comment for this PR as for the PR to master: #14602 (review)

This PR needs a few things before I'd consider it ready for merge:

• A rationale for why we'd want to provide this 'feature'. Summing up untrusted coins into a 'balance' can lead to very confusing results, such as negative balances and double-counting conflicting transactions. An uninformed user could see money 'disappear' or be tricked into thinking they've received money which they haven't.
• Full documentation of the usage. I removed getbalance * here: https://github.com/bitcoin/bitcoin/pull/12953/files#diff-df7d84ff2f53fcb2a0dc15a3a51e55ceL822 because the option had been marked as deprecated. If we want to support this usage, then it should be properly documented, with appropriate warnings and caveats.
• Full test coverage, so this doesn't get regressed again. I wrote some tests here: https://github.com/jnewbery/bitcoin/tree/untrusted_balance in the course of reviewing this PR.
• Not gratuitously breaking bitcoin-cli because you don't happen to use it.

Also for this backport:

• Not deleting linters because they fail on your branch(!)

[luke-jr]

The linter is broken by design.

[DrahtBot]

Needs rebase