Make KeyValueStore non-optional to avoid intermittent failure on load

We've seen multiple instances at one installation where this is happening
intermittently on startup:

...
Caused by: org.osgi.service.blueprint.container.ServiceUnavailableException: No matching service for optional OSGi service reference: (objectClass=org.opennms.integration.api.v1.distributed.KeyValueStore)
        at org.apache.aries.blueprint.container.ReferenceRecipe.getService(ReferenceRecipe.java:272) ~[?:?]
        at org.apache.aries.blueprint.container.ReferenceRecipe.access$000(ReferenceRecipe.java:57) ~[?:?]
        at org.apache.aries.blueprint.container.ReferenceRecipe$ServiceDispatcher.call(ReferenceRecipe.java:341) ~[?:?]
        at Proxy2eb34fe8_fa63_41d0_96b1_a2e6f7cd7d24.enumerateContextAsync(Unknown Source) ~[?:?]
        at org.opennms.timeseries.cortex.CortexTSS.<init>(CortexTSS.java:180) ~[?:?]
...

plugin/src/main/resources/OSGI-INF/blueprint/blueprint.xml

@@ -33,7 +33,7 @@
     </bean>
 
     <!--Key-value store -->
-    <reference id="keyValueStore" interface="org.opennms.integration.api.v1.distributed.KeyValueStore" availability="optional"/>
+    <reference id="keyValueStore" interface="org.opennms.integration.api.v1.distributed.KeyValueStore"/>
 
     <bean id="timeSeriesStorage" class="org.opennms.timeseries.cortex.CortexTSS" destroy-method="destroy">
         <argument ref="cortexTssConfig" />