Use grequests as test client.

   * grequests is based on requests and gevent.
   * Avoid process spawning overhead in previous test method
optimization by pre-filtered data frame on 'table'

   * Test using the benchmark.py in THIS COMMIT
   * Time: from 56s to 4.6s
   * Memory: 2x (one copy of pre-filtered DFs)
Merge pull request #61 from gazetteerhk/backopt

Backopt