Fix leaking handles in uncommitted forkers

[jasagredo]

No description provided.

[amesgen]

LGTM

Not in scope for this PR, but the story around safely and correctly releasing resources for forkers is quite complicated (even more so with this PR), with some parts being handled by a ResourceRegistry and some parts by a special StrictTVar m (m ()). I wonder whether we could make some general improvements here; maybe the concept of a temporary registry a la runWithTempRegistry could be used to streamline this?

[jasagredo]

I plan on merging this as a PR stack:

• Extract backends into separate packages #1653
• Version ledger tables and encode TxIns in big-endian #1652
• Support LSM in snapshot-converter and streaming ledger tables #1603
• LSM trees initial integration #1572
• Reorganize LedgerDB constraints #1644
• Return the last read key when doing range reads #1641
• Define SnapshotManager #1643
• Fix leaking handles in uncommitted forkers #1640 ←

This is a graph of the dependencies of the PRs

            ┌─────┐
            │#1653│
            └──┬──┘
               │
            ┌──▼──┐
            │#1652│
            └──┬──┘
               │
            ┌──▼──┐
            │#1603│
            └──┬──┘
               │
               │
            ┌──▼──┐
            │#1572│
            └─────┘
               │
   ┌───────┬───┴───┬───────┐
┌──▼──┐ ┌──▼──┐ ┌──▼──┐ ┌──▼──┐
│#1640│ │#1643│ │#1641│ │#1644│
└─────┘ └─────┘ └─────┘ └─────┘