fn: lb & pure-runner slot hash id communication (#1007)

* fn: lb & pure-runner slot hash id communication

With this change, LB can pre-calculate the slot hash
key and pass it to runners. If LB knows/calculates
the slot hash ids, then it can also make better
estimates on which runner can successfully execute
it especially when status messages from runner
include a small summary of idle slots for a given
slot hash id. (TODO)

* fn: fix mock test

api/agent/lb_agent_test.go

@@ -122,12 +122,17 @@ type mockRunnerCall struct {
 	rw         http.ResponseWriter
 	stdErr     io.ReadWriteCloser
 	model      *models.Call
+	slotHashId string
 }
 
 func (c *mockRunnerCall) LbDeadline() time.Time {
 	return c.lbDeadline
 }
 
+func (c *mockRunnerCall) SlotHashId() string {
+	return c.slotHashId
+}
+
 func (c *mockRunnerCall) RequestBody() io.ReadCloser {
 	return c.r.Body
 }