CBL-7717 : Lockmanager resources release on shutdown (#451)

* CBL-7717 : Lockmanager resources release on shutdown

* Resolving the race condition issue

* Going with setting RefQueueCleanerThread as daemon

* Removed unnecessary spaces and imports

Author: Aniket392

common/main/java/com/couchbase/lite/internal/core/peers/LockManager.java

@@ -106,6 +106,7 @@ public Object getLock(long ref) {
 
             if (refQCleaner == null) {
                 final RefQueueCleanerThread t = new RefQueueCleanerThread("LockManager cleaner", refQueue);
+                t.setDaemon(true);
                 refQCleaner = t;
                 t.start();
             }