Prune Cardano transactions stored on signer

[jpraynaud]

Why

In order to provide minimum footprint of the signer nodes on the SPO infrastructure, we want to keep stored only the minimum transactions needed to enable consistent signature.

What

Prune transactions which are older than the Cardano k security parameter (i.e number of blocks after which the blocks are immutable) and which have corresponding block range roots stored in the database.

Warning

The k parameter is a genesis parameter that may be different depending on the network. In a first implementation we will fix the parameter given the Cardano network for simplicity, and we will probably use the protocol parameters in a second implementation to get access to the value. The k value for mainnet is 2160.

How

• Implement a decorator for TransactionImporter trait that does the pruning of transactions in the signer
• Make prune transactions with block number that is strictly less than MAX(block_range_root.start) - k
• Make k security parameter (which represents the number of blocks to keep) configurable for the signer (with an value with default of 2160)