* use flushed last applied index in gc

Signed-off-by: qi.xu <tonxuqi@outlook.com>

Add a new config: server.max_snapshot_file_raw_size (size before compression) to limit the write amplification in big region scenario.
For scenarios when older version TiKv co-exists (like rolling upgrade), it will rely on feature gate and still use single snapshot file for backward compatibility.

Signed-off-by: tonyxuqqi <tonyxuqi@outlook.com>