Implement database connection pooling for Cardano transaction repository

[jpraynaud]

Why

We have noticed during performances optimization of the Cardano transaction prover that the connections to the SQLite database is a bottleneck. A first prototype showed throughtput increased by a factor of ~x10 when implementing resource pooling on the connections to the database. However, in order to be properly implemented, the resource pooling must be done in a setup where all write queries are implemented with SQL transactions.

What

Implement a connection pooling mechanism for the Cardano transaction repository and remove the performance bottleneck on the Cardano transaction prover.

How

• Implement a pooling mechanism for the Cardano transaction repository
• Add a configuration parameter for the connection pool size
• Use the connection pool size parameter in the devnet
• Use the connection pool size parameter in the terraform infrastructure
• Use the connection pool size parameter in the CI
• Set the GitHub Actions environment variables
• Update documentation
• Benchmark the performances improvements