Add Geary.Logging.clear method, call it on app shutdown

src/client/application/geary-application.vala

@@ -661,6 +661,8 @@ public class GearyApplication : Gtk.Application {
         }
 
         quit();
+
+        Geary.Logging.clear();
         Util.Date.terminate();
     }
 

src/engine/api/geary-logging.vala

@@ -350,6 +350,23 @@ public void init() {
     max_log_length = DEFAULT_MAX_LOG_BUFFER_LENGTH;
 }
 
+/**
+ * Clears all log records.
+ *
+ * Since log records hold references to Geary engine objects, it may
+ * be desirable to clear the records prior to shutdown so that the
+ * objects can be destroyed.
+ */
+public void clear() {
+    record_lock.lock();
+
+    first_record = null;
+    last_record = null;
+    log_length = 0;
+    
+    record_lock.unlock();
+}
+
 /**
  * Replaces the current logging flags with flags.  Use Geary.Logging.Flag.NONE to clear all
  * logging flags.