Use separate requests to write design when replicating

Design doc writes could fail on the target when replicating with non-admin
credentials. Typically the replicator will skip over them and bump the
`doc_write_failures` counter. However, that relies on the POST request
returning a `200 OK` response. If the authentication scheme is implemented such
that the whole request fails if some docs don't have enough permission to be
written, then the replication job ends up crashing with an ugly exception and
gets stuck retrying forever. In order to accomodate that scanario write _design
docs in their separate requests just like we write attachments.

Fixes: #2415

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