fix: canister log migration

[mraszyk]

This PR fixes canister log (data) migration:

• a new Boolean value migrated in canister.pbuf (inside log_memory_store in SystemState) tells if logs have been migrated from the (legacy) field canister_log in SystemState to the new field log_memory_store in SystemState:
  • default to false for existing canisters (transparently by protobuf since the field is currently missing);
  • set to the feature flag for new canisters (so that new canisters are not labeled as not migrated once the feature is enabled);
• fetch_canister_logs uses the new log_memory_store if the feature flag is true and the store is migrated;
• data migration: at the very beginning of SchedulerImpl::execute_round, for every canister
  • if the feature flag is true and the canister’s log memory store is not migrated => migrate (resize store to 4KiB – which is the maximum size of the legacy canister_log in SystemState –, migrate records, set migrated to true);
  • if the feature flag is false and the canister’s log memory store is migrated (i.e., after a downgrade) => “undo” migration (clear the log memory store by resizing down to 0 and reset migrated to false).

The log_memory_store_feature flag is used:

• to trigger data migration and rollback in SchedulerImpl::execute_round;
• to initialize the migrated field for newly created canisters;
• to select between the legacy canister_log and new log_memory_store when reading logs via fetch_canister_logs (this way, flipping the flag immediately falls back to the legacy canister_log).

The migrated field is used:

• to ensure that data migration is one-off;
• to determine the byte capacity for canister logs during execution;
• to determine if new canister logs should be appended to the new log_memory_store;
• to determine if canister logs should be read from the new log_memory_store;
• to determine if the new log_memory_store can be resized via update_settings;
• to determine if metrics should be recorded for the new log_memory_store.

[basvandijk]

rs/tests changes LGTM.