librados: read into user's bufferlist for aio_read

* The 'buf' argument to read() used to be passed into
  AioCompletionImpl, and the results would be copied back after
  reading. This is replaced with the creation of a static buffer of
  that buf.

* The pbl argument in AioCompletionImpl is removed.

The patch is tested against an application using librados. I've
assumed that 'pbl' in

aio_read( ...., pbl, )

is allocated by the user. It may even speed things up: a buffer copy
is prevented.

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>