Allow user to set clientContextId query option.

Author: dnault

couchbase-analytics-java-client/src/main/java/com/couchbase/analytics/client/java/QueryOptions.java

@@ -156,6 +156,7 @@ static class Unmodifiable {
     void injectParams(ObjectNode query) {
       query.put(
         "client_context_id",
+        // Generating the random ID late like this means a new ID is assigned to each retry.
         clientContextId != null ? clientContextId : UUID.randomUUID().toString()
       );