Return the last read key when doing range reads

[jasagredo]

No description provided.

[jasagredo]

I plan on merging this as a PR stack:

• Extract backends into separate sublibraries #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 ←

This is a graph of the dependencies of the PRs

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

[fraser-iohk]

my only minor taste-related concern is returning a tuple that's a bit contextless instead of some kind of data type with two fields, but otherwise happy with this