This is causing confusing panics in tailscale/corp#34485. We'll keep
using the tka.ChonkMem constructor as much as we can, but don't panic
if you create a tka.Mem directly -- we know what the sensible thing is.
Updates #cleanup
Signed-off-by: Alex Chan <alexc@tailscale.com>
Change-Id: I49309f5f403fc26ce4f9a6cf0edc8eddf6a6f3a4