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/call.go

@@ -315,12 +315,17 @@ type call struct {
 	execDeadline   time.Time
 	requestState   RequestState
 	containerState ContainerState
+	slotHashId     string
 }
 
 func (c *call) LbDeadline() time.Time {
 	return c.lbDeadline
 }
 
+func (c *call) SlotHashId() string {
+	return c.slotHashId
+}
+
 func (c *call) RequestBody() io.ReadCloser {
 	if c.req.Body != nil && c.req.GetBody != nil {
 		rdr, err := c.req.GetBody()