MDMF publisher sends duplicate write vector elements

[davidsarah]

The MDMF publisher (in mutable/layout.py) sends duplicate write vector elements for the start of the MDMF header. This confused the cloud backend before I fixed it to tolerate duplicates; in any case, the publisher probably shouldn't be doing that.

IRC log of debugging session:

the write vector that fails is: [(2167, 54), (123, 346), (2221, 32), (469, 136), (0, 41), (605, 66), (671, 93), (59, 64), (41, 18), (0, 41)]
(first element of each pair is data offset, second is length of write)
which contains two writes at offset 0 of length 41
***davidsarah looks at the MDMF publisher
the first 41 bytes is the container type byte (SDMF or MDMF),
   sequence number, and root hash
which suggests that MDMFSlotWriteProxy.put_root_hash is being
   called twice...
no, it isn't
so in that case, put_root_hash is being called, and the
  if not self._written branch in _write is also being taken, maybe
yep, that's it