Adds extension points for runner pools in load-balanced mode (Patch 1/2) (#851)

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

* adds error to return values in RunnerPool and Runner interfaces

api/models/runner_pool.go

@@ -0,0 +1,31 @@
+package models
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"time"
+)
+
+// 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() 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() 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() io.Writer
+	StdErr() io.ReadWriteCloser
+	Model() *Call
+}