Originally, ec duplication is designed to make duplicated copy at
chunk_transformer function for each node. However, it can cause doubled
(or more than, depends on ec_duplication_factor) etag calculation which
should be expensive for Swift proxy work.
Since this patch, chunk_transformer keeps only an unique fragments slot
to store, and then, send_chunk in ECObjectController._transfer_data
picks up the suitable chunk by unique backend fragment index assigned by
_determine_chunk_destination function to send to object server.
Note that, Putter still keeps node_index but the new putter_to_frag_index
dict and frag_hasher (chunk_index and chunk_hasher in the old names)
now refers the value by fragment index.
Change-Id: Ib9e8a6f67c2985164dd20b049c7f144f19fd1822
4187c839c3