skip removing entries from sqlite if store-queue-on-shutdown is used

if `--store-queue-on-shutdown` is used, database should only be accessed during
shutdown and on initial startup. Skipping the done() saves the unnecessary query
preparation, database locks, etc...

 - fixes #369

libgearman-server/plugins/base.h

@@ -147,6 +147,11 @@ class Context {
     _store_on_shutdown= store_on_shutdown_;
   }
 
+  bool is_store_on_shutdown()
+  {
+    return _store_on_shutdown;
+  }
+
   bool has_error()
   {
     return _error_string.size();

libgearman-server/plugins/queue/sqlite/instance.cc

@@ -432,6 +432,9 @@ gearmand_error_t Instance::done(gearman_server_st*,
                                    const char *function_name,
                                    size_t function_name_size)
 {
+  if (is_store_on_shutdown())
+    return GEARMAND_SUCCESS;
+
   gearmand_log_debug(GEARMAN_DEFAULT_LOG_PARAM,
                      "sqlite done: unique_key: %.*s, function_name: %.*s",
                      int(unique_size), (char*)unique,