Remove unsafe indexing from `src/message_pool/msg_chain.rs` and `src/message_pool/msgpool/selection.rs`

[lemmih]

Issue summary

Both src/message_pool/msg_chain.rs and src/message_pool/msgpool/selection.rs had code untouched for years. Now we're using the code again, which needs some love/cleaning. The modules have a lot of unsafe indexing, and they allow clippy::indexing_slicing.

Tasks:

• [ ] Remove #![allow(clippy::indexing_slicing)] from src/message_pool/msg_chain.rs.
• [ ] Remove #![allow(clippy::indexing_slicing)] from src/message_pool/msgpool/selection.rs.

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