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/agent/lb_agent_test.go

@@ -128,15 +128,18 @@ func (c *mockRunnerCall) SlotDeadline() time.Time {
 	return c.slotDeadline
 }
 
-func (c *mockRunnerCall) Request() *http.Request {
-	return c.r
+func (c *mockRunnerCall) RequestBody() io.ReadCloser {
+	return c.r.Body
 }
+
 func (c *mockRunnerCall) ResponseWriter() http.ResponseWriter {
 	return c.rw
 }
+
 func (c *mockRunnerCall) StdErr() io.ReadWriteCloser {
 	return c.stdErr
 }
+
 func (c *mockRunnerCall) Model() *models.Call {
 	return c.model
 }