Moves out node pool manager behind an extension using runner pool abstraction (Part 2) (#862)

* Move out node-pool manager and replace it with RunnerPool extension

* adds extension points for runner pools in load-balanced mode

* adds error to return values in RunnerPool and Runner interfaces

* Implements runner pool contract with context-aware shutdown

* fixes issue with range

* fixes tests to use runner abstraction

* adds empty test file as a workaround for build requiring go source files in top-level package

* removes flappy timeout test

* update docs to reflect runner pool setup

* refactors system tests to use runner abstraction

* removes poolmanager

* moves runner interfaces from models to api/runnerpool package

* Adds a second runner to pool docs example

* explicitly check for request spillover to second runner in test

* moves runner pool package name for system tests

* renames runner pool pointer variable for consistency

* pass model json to runner

* automatically cast to http.ResponseWriter in load-balanced call case

* allow overriding of server RunnerPool via a programmatic ServerOption

* fixes return type of ResponseWriter in test

* move Placer interface to runnerpool package

* moves hash-based placer out of open source project

* removes siphash from Gopkg.lock

Gopkg.lock

@@ -104,12 +104,6 @@
   revision = "8ab6407b697782a06568d4b7f1db25550ec2e4c6"
   version = "v0.2.0"
 
-[[projects]]
-  name = "github.com/dchest/siphash"
-  packages = ["."]
-  revision = "4ebf1de738443ea7f45f02dc394c4df1942a126d"
-  version = "v1.1.0"
-
 [[projects]]
   name = "github.com/docker/distribution"
   packages = [
@@ -659,6 +653,6 @@
 [solve-meta]
   analyzer-name = "dep"
   analyzer-version = 1
-  inputs-digest = "98644a27c50160c1daa8110197722573a586ea897d713e19525deccafffa44b3"
+  inputs-digest = "d8661bd78eda741b449a3675c52207af1bd738c63cb4f74d018edbaf0e9ef77d"
   solver-name = "gps-cdcl"
   solver-version = 1