tuned 2464ba6e6fc9 -- NB: approximative_id is NONE for PIDE document transactions;

signal work_available should be sufficient to initiate daisy-chained workers, and lead to separate broadcast work_finished eventually -- NB: broadcasting all worker threads tends to burn parallel CPU cycles;

less eventful shutdown: merely wait for scheduler to terminate;

tuned messages;

tuned;

merged;

more explicit Goal.shutdown_futures;