text: Title: fetchmany argument ignored?
  Body: I am using PyHive and reading from a table with 527,000 rows, which takes quite a long time to read.
  In trying to optimize the process, I found the following timings:
  
  fetchmany(1000) takes 4.2s
  fetchmany(2000) takes 8.4s
  fetchmany(500) takes 4.2s
  fetchmany(500) takes 0.02s if directly preceded by the other fetchmany(500)
  
  It seems like the batch size is 1000 regardless of the argument to fetchmany(). Is this the prescribed behavior? Is there an "under the hood" way to change this to optimize batched reads? Is there a way to "prefetch" so that data can be pipelined?
  
  thanks!
  