Avoid forcing the Ledger pulsers when serializing the Ledger State

Storing the ledger state on disk requires forcing the pulser, which consumes a significant amount of CPU time and memory, and raises garbage collection activity significantly. As a consequence, [slot leadership checks are missed](https://github.com/IntersectMBO/ouroboros-consensus/issues/868).

Possible ways to mitigate this are:

- Store a representation of the thunks involved in the pulser.
- Drop the information used by the pulser, and restore it when we load the ledger state from disk.