Tech debt: no pattern is used when dealing with cached data in irmaclient.Client

[ivard]

The irmaclient.Client struct currently caches various data, representing the data stored in irmaclient.storage. No specific pattern is followed in this caching process, making it difficult to detect errors in maintaining consistency between the cache and the storage.

Revocation poses a particular challenge because numerous updates are required when re-downloading the nonrevocation witness. These updates occur in background tasks. Currently, it is challenging to assess whether potential race conditions exist because strict module separation is not applied.

The cached storage is currently used, among other things, to calculate the choices the user currently has when starting a disclosure session. Presently, an update is sent for each session every time the cache is updated. This mechanism is fragile, and it would be better if each session kept its own administration and only received an event when updates occurred. In this sense, the 'sessions' struct is also considered a code smell.