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/runnerpool/runner_pool.go

@@ -41,6 +41,7 @@ type Runner interface {
 // RunnerCall provides access to the necessary details of request in order for it to be
 // processed by a RunnerPool
 type RunnerCall interface {
+	SlotHashId() string
 	LbDeadline() time.Time
 	RequestBody() io.ReadCloser
 	ResponseWriter() http.ResponseWriter