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/pure_runner.go

@@ -536,6 +536,9 @@ func (pr *pureRunner) handleTryCall(tc *runner.TryCall, state *callHandle) error
 	}
 
 	state.c = agent_call.(*call)
+	if tc.SlotHashId != "" {
+		state.c.slotHashId = tc.SlotHashId
+	}
 	state.allocatedTime = strfmt.DateTime(time.Now())
 	pr.spawnSubmit(state)