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/runnerpool/runner_pool.go

@@ -0,0 +1,39 @@
+package runnerpool
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"time"
+
+	"github.com/fnproject/fn/api/models"
+)
+
+// Placer implements a placement strategy for calls that are load-balanced
+// across runners in a pool
+type Placer interface {
+	PlaceCall(rp RunnerPool, ctx context.Context, call RunnerCall) error
+}
+
+// RunnerPool is the abstraction for getting an ordered list of runners to try for a call
+type RunnerPool interface {
+	Runners(call RunnerCall) ([]Runner, error)
+	Shutdown(context.Context) error
+}
+
+// Runner is the interface to invoke the execution of a function call on a specific runner
+type Runner interface {
+	TryExec(ctx context.Context, call RunnerCall) (bool, error)
+	Close(ctx context.Context) error
+	Address() string
+}
+
+// RunnerCall provides access to the necessary details of request in order for it to be
+// processed by a RunnerPool
+type RunnerCall interface {
+	SlotDeadline() time.Time
+	Request() *http.Request
+	ResponseWriter() http.ResponseWriter
+	StdErr() io.ReadWriteCloser
+	Model() *models.Call
+}