Add a configurable grace period (default 5s) that delays closing file
handles and downloaders when the last handle closes. If a new handle
opens within the grace period, it reuses the existing resources.
This fixes 40x performance degradation with serve nfs vs serve sftp
caused by go-nfs opening/reading/closing on every NFS READ RPC, which
destroyed read-ahead prefetch before it could accumulate.
The grace period only applies to non-dirty files so that writeback
proceeds immediately on close.
Fixes#9251
0457b0b375