Define SnapshotManager

[jasagredo]

No description provided.

[jasagredo]

I plan on merging this as a PR stack:

• 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
• Remove Shelley and Byron operation modes in cardano tools #1647

This is a graph of the dependencies of the PRs:

                 ┌─────┐       
                 │#1603│       
                 └──┬──┘       
                    │          
               ┌────┴─────┐    
            ┌──▼──┐    ┌──▼──┐ 
            │#1572│    │#1647│ 
            └─────┘    └─────┘ 
               │               
   ┌───────┬───┴───┬───────┐   
┌──▼──┐ ┌──▼──┐ ┌──▼──┐ ┌──▼──┐
│#1640│ │#1643│ │#1641│ │#1644│
└─────┘ └─────┘ └─────┘ └─────┘