Large memory usage when loading large `state` file

[noonio]

Context & versions

It has been reported that when loading a 170gig state file, the hydra-node uses lots of memory and indeed crashes. This should not be the case since #1618 has been merged.

Steps to reproduce

1. Create a large state file
2. Load it
3. Observe that memory goes up.

Actual behavior

Memory goes up.

Expected behavior

Memory usage should be bounded, even when reading from an arbitrarily-large state file.