Bugfix/grpc consume eof (#912)

* GRPC streams end with an EOF

The client should ensure that the final packet is followed by a GRPC
EOF. This has the benefit of permitting the client code to clean up resources.

* Don't require an entire HTTP request in RunnerCall

TryExec needs a handle on an incoming ReadCloser containing the body
of a request; however, everything else will already have been extracted
from the HTTP request in the case of lbAgent use.

(The point of this change is to simplify the interface for other uses.)

* Return error from GRPC layer explicitly

As per review

api/runnerpool/runner_pool.go

@@ -42,7 +42,7 @@ type Runner interface {
 // processed by a RunnerPool
 type RunnerCall interface {
 	SlotDeadline() time.Time
-	Request() *http.Request
+	RequestBody() io.ReadCloser
 	ResponseWriter() http.ResponseWriter
 	StdErr() io.ReadWriteCloser
 	Model() *models.Call