[BUG] RecordPageHandler initialize failed after observer restart.

Create a table and insert some rows, then stop observer.

➜  build ✗ ./bin/obclient -s miniob.sock
miniob > create table just_a_test (id int, id2 int);
SUCCESS
miniob > insert into just_a_test values (1, 1);
SUCCESS
miniob > insert into just_a_test values (2,2);
SUCCESS
miniob > exit
--- 
### then stop observer
➜  build ✗ ./bin/obclient -s miniob.sock
miniob > select * from just_a_test;
id | id2

Restart observer, then select from the table created before. It return no record.
RecordPageHandler will be nullptr and record_page_iterator_ is invalid.

RC RecordFileScanner::fetch_next_record()
{
  RC rc = RC::SUCCESS;
  if (record_page_iterator_.is_valid()) {
    // 当前页面还是有效的，尝试看一下是否有有效记录
    rc = fetch_next_record_in_page();
  .......
}

Reproduce the issue by creating rows, stopping and restarting the observer, then querying the table. Start at RecordFileScanner::fetch_next_record and inspect how RecordPageHandler and record_page_iterator_ are initialized after restart; done means the existing rows are returned instead of an empty result or invalid iterator.