Allow sending object metadata after data

This lets the proxy server send object metadata to the object server
after the object data. This is necessary for EC, as it allows us to
compute the etag of the object in the proxy server and still store it
with the object.

The wire format is a multipart MIME document. For sanity during a
rolling upgrade, the multipart MIME document is only sent to the
object server if it indicates, via 100 Continue header, that it knows
how to consume it.

Example 1 (new proxy, new obj server):

   proxy: PUT /p/a/c/o
          X-Backend-Obj-Metadata-Footer: yes

     obj: 100 Continue
        X-Obj-Metadata-Footer: yes

   proxy: --MIMEmimeMIMEmime...

Example2: (new proxy, old obj server)

   proxy: PUT /p/a/c/o
          X-Backend-Obj-Metadata-Footer: yes

     obj: 100 Continue

   proxy: <obj body>

Change-Id: Id38f7e93e3473f19ff88123ae0501000ed9b2e89