Extract backends into separate packages

[jasagredo]

Description

Please include a meaningful description of the PR and link the relevant issues
this PR might resolve.

Also note that:

• New code should be properly tested (even if it does not add new features).
• The fix for a regression should include a test that reproduces said regression.

[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│
└─────┘ └─────┘ └─────┘ └─────┘