Avoid nullptr derefer when constr Blockchain & tx_memory_pool (Cont.)

[jeffro256]

This PR replaces #8033 because it seems to be abandoned. Mine follows the same principle, but I extended the PR to enforce the usage of BlockchainAndPool throughout the codebase by making the constructors for Blockchain and tx_memory_pool private.

[jeffro256]

@UkoeHB

[UkoeHB]

Thanks

[selsta]

@jeffro256 can you add DarkWingMcQuack as an author in git for the first commit?

[jeffro256]

He should be the sole author of the first commit already, no?

[jeffro256]

If you checkout the branch and do git log you see Author: lukas <np-hardass@protonmail.com>.

[selsta]

@jeffro256 sorry, it showed your profile picture that's why i got confused :)

[jeffro256]

Sorry fixed a really small license year typo

[vtnerd]

Did you consider using a std::shared_ptr instead? Or is the churn on that massive? It's would likely be annoying because one of them would have to be a std::weak_ptr to break the cycle, causing more code churn.

This patch isn't perfect, but I think it does improve the code quality a bit.

[jeffro256]

One benefit of not using pointers is that you can allocate these objects on the stack, removing one layer of indirection.