tka: implement filesystem-based tailchonk implementation

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>
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