Implement invariants

[RiccardoM]

Currently there are no invariants set for the posts module nor the magpie module. I really think we should implement those as they allow to check for each block that the state is preserved correctly.

[RiccardoM]

Some possible invariants that might be useful to preserve are the following:

• Make sure the LastPostID is always equals to the greatest PostID inserted.
  This should avoid the reproducing of Duplicate entries in children after post edit #63 in the future

• Make sure that each post reaction refers to an existing post.
  This should avoid storing invalid reactions.

• Make sure each post has a ParentID referring to an existing post.
  This should avoid storing invalid comments.

[leobragaz]

Also maybe:
Make sure that pollAnswers referring to an existing post with poll?

[RiccardoM]

If we change how ids are generated (#131), we could change the comments check to be:

• Make sure that every post comment has a creation date after the parent post

[leobragaz]

We could start implementing invariants after we fixed posts IDs

[RiccardoM]

@bragaz Yeah I think that's the way to go. Maybe we can have id fixing into a PR and invariants into another one, both targeting 0.4.0? Do you think that could be possibile?

[leobragaz]

This depends on when we decided to roll-out the new testnet.
I think it would be better anyway to target new IDs fixes to 0.4.0 and the invariants to 0.5.0.
This should give us the proper time to test them carefully.
But, if you think this should better added into 0.4.0 we can make it.