Audit memory usage

@CodiePP reports that the node uses 1.25 GB memory after an hour. It also has an impact on chain sync speed: it slows down from the initial ~500 blocks/s to ~250 blocks/s.

It would surprise me if it were a space leak as in "we have unforced thunks in our state", because the `NoUnexpectedThunks` checks pass (unless we're not checking some other forms of state). Maybe we're simply holding on to more and more memory? For example, the ledger will grow. We should investigate.