Translib APIs for subscription (sonic-net#92)

Added/updated following translib APIs:
 - Subscribe() for handling ON_CHANGE notifications
 - Stream() for handling SAMPLE, POLL, ONCE subscriptions
 - IsSubscribeSupported() for collecting subscription capabilities
 - NewSubscribeSession() to create a SubscribeSession object

These APIs use the new appInterafce functions translateSubscribe() and
processSubscribe() to process subscription request and responses.

Signed-off-by: Sachin Holla <sachin.holla@broadcom.com>

translib/db/subscribe.go

@@ -41,6 +41,7 @@ type SKey struct {
 	Ts  *TableSpec
 	Key *Key
 	SEMap map[SEvent]bool	// nil map indicates subscribe to all
+	Opaque interface{}      // opaque data set by the caller
 }
 
 type SEvent int

translib/path/path_validator.go

@@ -280,8 +280,8 @@ func (pv *pathValidator) validateListKeyValues(schema *yang.Entry, gPath *gnmi.P
 		log.Warningf(errStr)
 		return tlerr.NotFoundError{Format: fmt.Sprintf("Invalid key present in the node path: %v; error: %s", gPath, errStr)}
 	}
-	if log.V(4) {
-		pretty.Print(ygotStruct)
+	if log.V(6) {
+		log.Infof("ygotStruct = %s", pretty.Sprint(ygotStruct))
 	}
 	if err := ygotStruct.Validate(&ytypes.LeafrefOptions{IgnoreMissingData: true}); err != nil {
 		errStr := fmt.Sprintf("error in ValidatedGoStruct.Validate: %v", err)