Added optional support for request streaming, closes getlantern/lantern#1767

It's not clear how much of an improvement this will be.  Before and after test results below:

### Before
```bash
➜  ox.to.a.cart  boom -allow-insecure -n 1000 -c 10 -q 100 -x http://localhost:10080 https://www.google.com/humans.txt
1000 / 1000 Boooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo! 100.00 % 

Summary:
  Total:	62.8833 secs.
  Slowest:	5.4758 secs.
  Fastest:	0.5246 secs.
  Average:	0.6260 secs.
  Requests/sec:	15.9025

Status code distribution:
  [200]	1000 responses

Response time histogram:
  0.525 [1]	|
  1.020 [962]	|∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  1.515 [26]	|∎
  2.010 [1]	|
  2.505 [0]	|
  3.000 [0]	|
  3.495 [0]	|
  3.990 [6]	|
  4.486 [1]	|
  4.981 [1]	|
  5.476 [2]	|

Latency distribution:
  10% in 0.5348 secs.
  25% in 0.5430 secs.
  50% in 0.5544 secs.
  75% in 0.5701 secs.
  90% in 0.6613 secs.
  95% in 0.8907 secs.
  99% in 3.6958 secs.
```

### After
```bash
➜  ox.to.a.cart  boom -allow-insecure -n 1000 -c 10 -q 100 -x http://localhost:10080 https://www.google.com/humans.txt
1000 / 1000 Boooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo! 100.00 % 

Summary:
  Total:	61.7948 secs.
  Slowest:	4.8525 secs.
  Fastest:	0.5229 secs.
  Average:	0.6149 secs.
  Requests/sec:	16.1826

Status code distribution:
  [200]	1000 responses

Response time histogram:
  0.523 [1]	|
  0.956 [960]	|∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
  1.389 [27]	|∎
  1.822 [1]	|
  2.255 [1]	|
  2.688 [2]	|
  3.121 [0]	|
  3.554 [3]	|
  3.987 [3]	|
  4.420 [1]	|
  4.852 [1]	|

Latency distribution:
  10% in 0.5348 secs.
  25% in 0.5428 secs.
  50% in 0.5528 secs.
  75% in 0.5701 secs.
  90% in 0.6596 secs.
  95% in 0.8202 secs.
  99% in 2.3905 secs.
```
> The reason that we have to close the body is that CloudFlare (and potentially other CDNs) will buffer the request on their end, and won't send it upstream to our proxy until the request is finished.

And "finished" means closed here correct? That's so surprising just given HTTP 1.1 persistent connections. I wonder how that ever works with just a normal client that doesn't close those connections?



> In the case of a large upload, we exceed CloudFlare's buffer and it actually starts streaming the data to our proxy.

Ah interesting. And we don't close on our end because we're getting a steady stream of write data. Makes sense.