streaming attachments writes

[Damien Katz]

I checked in streaming attachment writes for attachment uploads (ie  
PUT /db/docid/attachment.txt ...). This allows that huge files can be  
uploaded without CouchDB buffering it to memory, making is possible to  
upload huge attachments.

Unfortunately, we don't yet stream the attachments for replication w  
and you can't update an attachment and the document json in single  
request yet, so this is of limited use for now. That will require  
sending documents in http multi-part, and support for that is on the  
to-do list.

-Damien