1. When file meta cache is disabled (by setting `max_external_file_meta_cache_num=0` in be.conf),
the parquet's meta info is owned by parquet reader and will be released when calling `reader->close()`.
But the underlying file reader of this parquet reader will be released after `reader->close()`,
this may causing `heap-use-after-free` bug because some part of meta info may be referenced by file reader.
This PR fix it by making sure that meta info is released after file reader released.
2. Add modification time for file meta cache in BE, to avoid parquet read error like:
`Failed to deserialize parquet page header`
3a2c0d16f7