Add auto pruning in stores

[jpraynaud]

Issue

In order to keep stores with low overhead, we need to prune some stores of the Aggregator/Signers (depending on business rules)

Tasks

In Aggregator:

• Protocol Parameters (5 last epochs)
• Single Signatures (5 last epochs)
• Verification Keys (5 last epochs)
• Stake (5 last epochs)

In Signer:

• Protocol Initializer (5 last epochs)
• Stake (5 last epochs)

Documentation:

• Aggregator
• Signer

[ghubertpalo]

The retention period should be configurable per environment so dev & testing env could keep data for debug purposes.