Add 'invalidateblock' and 'reconsiderblock' RPC commands.

These can be used for testing reorganizations or for manual intervention in case of
chain forks.
Delay writing block indexes in invalidate/reconsider
Introduce a hidden category
Move `setmocktime` to hidden category

Another testing-only potential footgun command.
Merge pull request #5316

f86a24b Move `setmocktime` to hidden category (Wladimir J. van der Laan)
bd9aebf Introduce a hidden category (Pieter Wuille)
0dd06b2 Delay writing block indexes in invalidate/reconsider (Pieter Wuille)
9b0a8d3 Add 'invalidateblock' and 'reconsiderblock' RPC commands. (Pieter Wuille)