Feature/soledad adbapi3

DO NOT MERGE YET
just for testing / pullreq purposes

Sorry, now I get it, we have a `sync_thread_pool` that executes one sync task at a time. I think this would work fine for preventing concurrent syncs, but maybe would not work right for stopping the sync. On `stop_sync()` below, if you defer to the same sync pool that is already occupied with syncing, then you'd not have the effect of cancelling the current sync, as the stop command would only be executed after the sync has finished. Am I right, now?