Disable hashing and increase buffer size in perfdiag.

* Disable hashing during perfdiag downloads.
* Precompute the MD5 of each test file so it doesn't get computed during
  perfdiag uploads.
* Increase the buffer size for the s3 Key object from 8KB to 16KB. Note that I
  didn't use the key_buffer_size boto config option, because it would have to
  be set before importing the Key object, which can only be done program-wide
  not for a specific command. Instead, I just overwrote the Key object's
  attribute inside perfdiag.