Changes

Summary

1. proper dummy timing entries;
2. use only finished sessions in timing data;
3. tuned;
4. performance tuning for build schedule: explicit schedule generation, without mixing heuristics;
5. performance tuning for build schedule: faster stopping;
6. performance tuning for timing heuristic: pre-calculate graph operations;
7. move timing data into scheduler for more efficient heuristics (e.g., with pre-calculated values);
8. clarified signature: emphasize mutable instance;
9. clarified signature: more operations;
10. support for explicit SSH hostname;
11. proper local host (amending 62d7ef1da441);