Currently the swift client retries establishing a connection to the server
(by default up to 5 times). However, when downloading an object, once
the connection has been established and the inital headers have been
returned, no attempt is made to retry.
So, for example, if 99MB of a 100MB object have been downloaded and the
connection is then lost, the download will fail. This patch changes the
behaviour to re-establish the connection and fetch the remaining bytes
using the 'Range' header to offset.
Data retry is not yet supported if the original request is for a subset
of the object data (ie uses the 'Range' header), or if resp_chunk_size
has not been set.
The object's etag is checked using If-Match to make sure the object data
hasn't changed since the start of the download.
Change-Id: Iab47f10081ff39f6d344dbc2479cbc3bfd1c5b29