- Avoid more than one round-trip call when loading blobs via copy from
  the server.

- Avoid loading large amounts of blob data into memory.  The old
  storeBlob implementation was likely to queue blob adta faster than
  it could be sent, leading to a large memory foot print for the
  queue. Now, iterators are used to read data from files only when the
  network layer is ready to send it.

- Fixed storeBlob to move the input file to the blob cache (when not
  sharing the blob directiry with the server).

- Extended the loadBlob locking model to work with multiple processes
  by using file locks rather than threading locks.  A common
  configuration is to use a client process per core, so that a machine
  is likely to have many client processes and it should be possible
  for the client processes to share a common blob cache.