Executor abstraction (#384)

This change adds a layer of abstraction over poolmgr. Poolmgr is now just one of the ways to turn a function into a service; other implementations will be added. The executor abstraction is a uniform API over all these implementations.

* Executor layer added on top of pool manager

* Removed the external server for executor

* Minor changes to keep existing semantics as much possible

* Separating the executor vs. poolmgr backend functionality and associated data members

* Executor logic separated from Poolmgr backend completely, placeholder for new backend

* Changed references to poolmgr in tests

* Moved poolmgr to it's package, as a side effect moved Cache to its's package (was causing cyclical dependency) and had to make some data structures exposed outside package

* Rebased from master and changed references to tpr -> crd

* Executor layer added on top of pool manager

* Executor logic separated from Poolmgr backend completely, placeholder for new backend

* Changed podName to a generic objectReference in fscache (#391)

Changed podName to a generic objectReference in function service cache implementation.

* Moved poolmgr to it's package, as a side effect moved Cache to its's package (was causing cyclical dependency) and had to make some data structures exposed outside package

* Rebased from master and changed references to tpr -> crd

* Merged from master with latest changes

* Removed stale executor service & deployment from previous merge

* Addressed review comments, still testing some areas

router/router.go

@@ -51,7 +51,7 @@ import (
 	"github.com/gorilla/mux"
 
 	"github.com/fission/fission/crd"
-	poolmgrClient "github.com/fission/fission/poolmgr/client"
+	executorClient "github.com/fission/fission/executor/client"
 )
 
 // request url ---[mux]---> Function(name,uid) ----[fmap]----> k8s service url
@@ -71,17 +71,20 @@ func serve(ctx context.Context, port int, httpTriggerSet *HTTPTriggerSet, resolv
 	http.ListenAndServe(url, handlers.LoggingHandler(os.Stdout, mr))
 }
 
-func Start(port int, poolmgrUrl string) {
+func Start(port int, executorUrl string) {
 	fmap := makeFunctionServiceMap(time.Minute)
 
 	fissionClient, _, _, err := crd.MakeFissionClient()
 	if err != nil {
 		log.Fatalf("Error connecting to kubernetes API: %v", err)
 	}
+
 	restClient := fissionClient.GetCrdClient()
-	poolmgr := poolmgrClient.MakeClient(poolmgrUrl)
-	triggers, _, fnStore := makeHTTPTriggerSet(fmap, fissionClient, poolmgr, restClient)
+
+	executor := executorClient.MakeClient(executorUrl)
+	triggers, _, fnStore := makeHTTPTriggerSet(fmap, fissionClient, executor, restClient)
 	resolver := makeFunctionReferenceResolver(fnStore)
+
 	log.Printf("Starting router at port %v\n", port)
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()