Track ctimes internally

[jmmv]

sandboxfs currently does not track ctimes for the nodes it manages. Instead, it relies on the ctimes returned by the underlying file systems and propagates those to the caller.

This has unfortunate consequences: because we don't track ctimes, we cannot synthesize what the attributes of a file should be e.g. immediately after creation, which means we end up issuing extra stat calls in cases where we shouldn't.

So: track ctimes (and possibly also creation times) and avoid these unnecessary operations.