datastore: use cached snapshot time string in path

datastore: use cached snapshot time string in path

When getting the `full_path` of a snapshot we did not use the cached
time string. By using it we avoid a call to the super-slow libc strftime.

This has some minor performance improvements of circa 7%. That is ~100ms
on my datastore with ~5000 snapshots.

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>