Handle open_revs retries at a higher level

This patch disables the httpc client retries for the request to stream
document revisions to the replicator.  The retry logic at that level
could end up jumbling together response body data from different
requests and thoroughly confusing the multipart parser.  Moving the
retry logic up a level allows us to start fresh each time.

BugzID: 21367
Ensure the att reader learns of a Parser death
Disable automatic retries of streaming writes

An automatic retry makes no sense here, as we've already streamed part
of the response body from the GET.  Disabling the retries bubbles the
error up to run_user_fun inside open_doc_revs allows us to retry the
whole GET + PUT correctly.

BugzID: 20822
Simplify doc_streamer initialization

The unlink at the end is a noop, so we can simplify this to use
spawn_link/3 and the fully-exported function call instead of
spawn_link/1 and a closure.

BugzID: 20822
Merge pull request #36 from cloudant/21367-multipart-retries-break

Ensure multipart/mixed operations are retried atomically

BugzID: 21367