Ensure multipart/mixed reads are retried atomically

This patch eliminates the use of couch_replicator_httpc's retry mechanism for multipart/mixed streaming reads and writes.  That mechanism had the nasty behavior of jumbling together response data from different requests and thoroughly confusing the multipart parser.  Instead we wrap the whole mechanism in a process and restart all of it if necessary.

BugzID: 21367
BugzID: 20822