FS implements Chonk, and given the expected load characteristics (frequent use
of AUM() + ChildAUMs(), and infrequent use of Heads() + CommitVerifiedAUMs()), the
implementation avoids scanning the filesystem to service AUM() and ChildAUMs().
Signed-off-by: Tom DNetto <tom@tailscale.com>
Chonks are responsible for efficient storage of AUMs and other TKA state.
For testing/prototyping I've implemented an in-memory version, but once we
start to use this from tailscaled we'll need a file-based version.
Signed-off-by: Tom DNetto <tom@tailscale.com>