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

api/agent/call.go

@@ -274,6 +274,22 @@ type call struct {
 	disablePreemptiveCapacityCheck bool
 }
 
+func (c *call) SlotDeadline() time.Time {
+	return c.slotDeadline
+}
+
+func (c *call) Request() *http.Request {
+	return c.req
+}
+
+func (c *call) ResponseWriter() http.ResponseWriter {
+	return c.w.(http.ResponseWriter)
+}
+
+func (c *call) StdErr() io.ReadWriteCloser {
+	return c.stderr
+}
+
 func (c *call) Model() *models.Call { return c.Call }
 
 func (c *call) Start(ctx context.Context) error {