is update_docs(_, _, _, false) idempotent?

[Adam Kocoloski]

Jason and I were working through COUCHDB-197 and discussing whether to  
retry failed HTTP requests during replication.  GET requests are  
surely safe to retry, but after I thought about it for awhile it  
seemed like the POSTs that occur during replication are also safe.

The POST to _missing_revs on the target server clearly does not change  
the state of the server; I assume it's a POST because the request has  
a body.  _bulk_docs is not normally an idempotent operation, but when  
"new_edits":false is sent along with the docs it looks like it might  
be, since no new revisions are generated.

Adam