9701: Changes on the Python SDK to allow small file packing on Collection class:

9701: Changes on the Python SDK to allow small file packing on Collection class:
* Added optional flush parameter to ArvadosFileWriter.close().
* Added _closed attribute & related accessors to ArvadosFile to enable BlockManager to query this state.
* For every ArvadosFile close() operation, call BlockManager to search for small blocks for repacking purposes.
* Do a last repacking operation just before exiting, joining the last small blocks even if they don't meet the
  minimum size requirement.

There's some pending code cleanup, as BlockManager fiddles directly with ArvadosFile._segments lists, some locking:
issues have to be solved.