converted main session manager to Consumer_Thread: messages need to be consumed immediately, postponed_changes replaces implicit actor mailbox scanning;

simplified commands_changed_buffer (in contrast to a8331fb5c959): rely on better performance of Consumer_Thread/Mailbox and more direct Timer (like session_actor.receiver);

simplified -- prefer Consumer_Thread over Actor;

tuned imports;

support for requests with explicit acknowledgment (and exception propagation);

more robust thread: continue after failure;