Add -initmempool setting

[maflcko]

The mempool is required for the Bitcoin network to function properly. Miners use it to select transaction for the next block and non-miner network nodes use it to accept/reject txs for relay.

However, if the node is not a miner nor wishes to participate in tx-relay, the mempool can be omitted. This hardens the node operation because less code runs in production and the node may be less susceptible to DoS attacks. For example, a user that is only interested in confirmed transactions, may wish to disable the mempool.

Moreover, the new setting encourages modularization within the software project.

A new boolean -initmempool setting should be added (default: True) to allow the mempool to be never initialized.

Useful skills:

• C++ (unique_pointers)
• Bitcoin Core codebase (init.cpp, ...)

Want to work on this issue?

For guidance on contributing, please read CONTRIBUTING.md before opening your pull request.

[maflcko]

To break this down into smaller tasks:

• Get familiar with init.cpp settings AddArg
• Get familiar with the task by looking at a recent change that was required for this project: https://github.com/bitcoin/bitcoin/pull/22415/files
• See where the mempool is passed and where guards (if checks) need to be added. This is needed because C++ doesn't check for nullptr dereference at compile time. At runtime it is UB and it is detectable with sanitizers (Asan or valgrind). For example Assert(node.mempool) or *node.mempool may need to be adjusted with guards.
• Run git grep '\<node.mempool' to find instances all instances in the codebase that may need fixups.