Fix max recursion for certain stackitem setups

[ixje]

What current issue(s) does this address, or what feature is it adding? close #1000

How did you solve this problem? implemented "reference" to self checking

How did you make sure your solution works? audit of mentioned testnet block now passes

Are there any special changes in the code that we should be aware of? no

Please check the following, if applicable:

• [ ] Did you add any tests?
• [X] Did you run make lint?
• [X] Did you run make test?
• [X] Are you making a PR to a feature branch or development rather than master?
• [X] Did you add an entry to CHANGELOG.rst? (if not, please do)

[ixje]

Don't merge until extra audit sync. Atm only validated against the last 25K blocks.

[ixje]

Syncing from 0 showed that block 723205 failed the audit because of a wrong equality implementation for CoinReference that is now called upon. The above commit fixes this and also addresses possible other cases. Running a longer sync

[ixje]

Going to call it a pass. Synced from 0 to 2961000 all blocks verified ok against neo-cli 2.10.2.