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.go

@@ -128,6 +128,7 @@ func (a *lbAgent) GetCall(opts ...CallOpt) (Call, error) {
 	c.req = c.req.WithContext(ctx)
 
 	c.lbDeadline = time.Now().Add(time.Duration(c.Call.Timeout) * time.Second)
+	c.slotHashId = getSlotQueueKey(&c)
 
 	return &c, nil
 }