[SVCS-899] Remove cute trick to deal with buffer size

[NyanHelsing]

Ticket

https://openscience.atlassian.net/browse/SVCS-899

Purpose

Entire files were being pulled into memory when the client's connection to waterbutler was faster than waterbutler's connection to the provider. This is less than ideal. Wrapping around a socket is cute, but it's not really the right way to do it. Put an actual limit on the buffer size and wait for it to get written. Sending it through a unix socket was a quick and dirty way to get some control into the tornado's data_received handler method, letting us wait until some of the data had been read; The StreamReader has some functionality built in around limits to the buffer size, but if it can't pause the coroutine that's writing things to it, it can't do anything with the data other than save it to it's buffer anyway, otherwise it loses data.

The RequestStreamReader can be tweaked a bit to handle writes to its buffer with a future, which would let it achieve the same thing without needing to wrap a streamreader in another streamreader, and without needing to open up a socketr pair.

The commit that added this trick: bb77d79

Changes

Side effects

QA Notes

Deployment Notes

[NyanHelsing]

The test failing shouldn't matter because we're removing v0.