os/bluestore: simplify BufferSpace writing_map / list

os/bluestore: simplify BufferSpace writing_map / list

We were tracking buffers by seq.  In reality, though, we
won't have very many buffers that have in flight IO to
the same blob at the same time, so we can get by with
just a list here.  We can rely on the fact that the
list will be sorted (seq's increase, and we put new
buffers at the end).

This saves us a memory allocation.  No change in size of
BufferSpace and Blob (std::list -> intrusive::list).

Signed-off-by: Sage Weil <sage@redhat.com>