code(chore): Test mempool `Msg::Reap{..}` handling is wrong

[ancazamfir]

When a proposal is built, transactions should not be removed from the mempool as the proposal may not be accepted. Transactions should be removed only after the proposal is decided, during app commit phase.
We need to somehow keep track of how many transactions have been reaped and skip them in subsequent 'reap: loop iteration.
maybe add something like:

    TakeFrom { 
        start_index: usize, 
        max_count: usize,
        reply: RpcReplyPort<(Vec<RawTx>, usize)> // (transactions, next_start_index)
    },

Or add a streaming iterator that can work with fifo mempool like our test impl