prefetching support

[heyitsanthony]

Fixes #39

[heyitsanthony]

Ran zkboom create (one connection, one etcd server, 10k keys, 8/8 key/val bytes) on my desktop with HD. Roughly 1.20x speedup.

Before prefetch:

Summary:
  Total:        8.5842 secs.
  Slowest:      0.0088 secs.
  Fastest:      0.0007 secs.
  Average:      0.0008 secs.
  Stddev:       0.0003 secs.
  Requests/sec: 1164.9350

Response time histogram:
  0.0007 [1]    |
  0.0015 [9602] |∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  0.0023 [375]  |∎
                       0.0031 [5]       |
  0.0039 [6]    |
  0.0047 [3]    |
  0.0055 [2]    |
  0.0063 [1]    |
  0.0072 [1]    |
  0.0080 [1]    |
  0.0088 [3]    |

Latency distribution:
  10% in 0.0007 secs.
  25% in 0.0007 secs.
  50% in 0.0008 secs.
  75% in 0.0008 secs.
  90% in 0.0010 secs.
  95% in 0.0014 secs.
  99% in 0.0017 secs.
  99.9% in 0.0046 secs.

After prefetch:

Summary:
  Total:        7.1183 secs.
  Slowest:      0.0087 secs.
  Fastest:      0.0005 secs.
  Average:      0.0007 secs.
  Stddev:       0.0003 secs.
  Requests/sec: 1404.8306

Response time histogram:
  0.0005 [1]    |
  0.0013 [9635] |∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  0.0022 [337]  |∎
                       0.0030 [7]       |
  0.0038 [7]    |
  0.0046 [1]    |
  0.0054 [3]    |
  0.0063 [3]    |
  0.0071 [3]    |
  0.0079 [1]    |
  0.0087 [2]    |

Latency distribution:
  10% in 0.0006 secs.
  25% in 0.0006 secs.
  50% in 0.0006 secs.
  75% in 0.0006 secs.
  90% in 0.0010 secs.
  95% in 0.0012 secs.
  99% in 0.0015 secs.
  99.9% in 0.0053 secs.

[xiang90]

lgtm