archive-push refactoring

[gsmolk]

Done:

1. multi-threading support
2. O_EXCL in remote mode
3. checksum computation on the remote agent
4. Move from "failing forward" approach
5. Improve overall robustness
6. --no-sync flag support
7. --no-ready-rename flag added
8. --batch-size flag added
9. --archive-timeout option support
10. print time elapsed (ms precision)

ToDo:

1. tests
2. documentation
3. help