Make Cardano transactions preloading infinite in signer

[jpraynaud]

Why

We want to avoid having signature blocked during the activation of the certification of the Cardano transactions on release-mainnet. This can happen for nodes already running when the activation is done: at the following pending certificate for the signed entity type, the signer will start importing the transactions and will likely get stuck in that state for the next ~5-6 hours. In order to circumvent the problem, we need the preloading of the signer (occurring on a background thread) to be launched between the activation of the feature and the first pending certificate created by the aggregator for that signed entity type (which will occur with ~6h delay after activation).

What

Make the Cardano transactions preloader infinitely repeat with a configurable repeat interval in the signer

How

• Implement infinite preloading of the Cardano transactions in the signer