Use latest=true when retrieving revisions

This allows the replicator to keep up with revisions that may have
occured after missing_revs was called.

BugzID: 14241
Avoid deadlocking the httpc pool

The multipart mime parsing code was getting into a state where it would
orphan parsers that held onto pool connections. This just adds a monitor
to make sure that the parser process dies when the parent dies normally
without finishing reading the rest of the parser.

BugzID: 16751
Fix replication deadlock after HTTP retries

An HTTP failure in the open_doc_revs can lead to a replication deadlock
if the retry happens while parsing the multipart/mime response. The
UserFun callback can end up attempting to PUT a request using an
attachment reader from the failed request while the pid that invoked
open_doc_revs will never see the started_open_doc_revs message which
indicates a retry has happened.

This just spawns a middleman process so that the process that invoked
open_doc_revs can listen for the response from the user function as well
as the started_open_doc_revs message to handle retries appropriately.
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
Be a bit nicer about logging
Bubble up missing_stub exceptions for proper retry

A missing_stub exception is special in that the replicator retries the
request without the atts_since optimmization.

COUCHDB-1901
[doc] fix commit ref
Merge branch '1901-atomic-multipart-retries' of https://github.com/apache/couchdb

Merged after r+ from rnewson on IRC, general trustworthiness of process.
Fix changelog formatting, order.
Add note about COUCHDB-1901 to changelog.