Change sequential stream to parallel stream for link discovery

If some devices are late to response result in to link discovery, remained switches's information will be pended to update.
So, to reduce the execution time and to prevent delay of update information, change sequential stream to parallel stream.

Change-Id: I4fb8dfd66b7e784d160994f87ffeee47b1a497aa
diff --git a/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/OsgiPropertyConstants.java b/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/OsgiPropertyConstants.java
index 4335fac..1ead70a 100644
--- a/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/OsgiPropertyConstants.java
+++ b/providers/link/src/main/java/org/onosproject/provider/linkdiscovery/impl/OsgiPropertyConstants.java
@@ -29,4 +29,7 @@
     public static final String POLL_FREQUENCY_SECONDS = "linkPollFrequencySeconds";
     public static final int POLL_FREQUENCY_SECONDS_DEFAULT = 10;
 
+    public static final String LINK_DISCOVERY_TIMEOUT_SECONDS = "linkDiscoveryTimeoutSeconds";
+    public static final int POLL_DISCOVERY_TIMEOUT_DEFAULT = 300;
+
 }