chunked upload ranges should increment by read data len
chunked upload ranges should increment by read data len

 * stream.read(*foo*) reads /up to/ *foo* bytes, not /exactly/ *foo*
   bytes.  We can try to read the maximum chunk size (and may succeed
   for local uploads) but will not necessarily get that much data.

   Increment the next read offset by the number of bytes of data
   actually read, rather than the maximum we can read.

CenterForOpenScience/waterbutler,felliott/waterbutler