Parallelized reading and writing

Resolves getlantern/lantern#1593
@myleshorton This should prevent the slow reader from being a problem.  This Write call is writing into one end of a pipe that is being read for sending to the upstream proxy (see reqBody [here](https://github.com/getlantern/enproxy/pull/2/files#diff-6fa614b63c77bc02aa9f275399c07d34R52)).  If the reader slows down, the writer slows down.  I tested this by simply putting in a sleep before the call to doRequest(), and the upload just kind of hangs and memory in the flashlight process doesn't grow.