Streaming request body into memory could OOM powerstrip with large build contexts

[lukemarsden]

Currently powerstrip's twisted reverse proxy implementation buffers the request in memory even if powerstrip knows it's not going to try and post it to any pre-hooks (e.g. docker build which streams an application/tar content-type in the request body).

Maybe this is already fixed in the golang port @progrium? In which case this issue can be closed as soon as the golang version replaces the twisted version.