tka: avoid quadratic filesystem scans

Since #17567, FS.ChildAUMs scans and decodes every active AUM file.
Authority reconstruction and compaction call it repeatedly, making
filesystem work quadratic in the number of AUMs.

Build per-FS indexes of active AUM hashes and parent-to-child
relationships in one scan. Use the indexes for graph queries while
continuing to decode returned AUM values from disk. Invalidate the
indexes after mutations, but retain them for empty purges.

A production-sized 825-AUM benchmark improves from 124 seconds to
4 seconds on macOS with Defender and from 5.9 seconds to 34 milliseconds
on Linux ext3. The checked-in benchmark exercises the 2,000-AUM maximum
linear history accepted by the traversal guardrail.

RELNOTE: Avoid Tailnet Lock startup failures with large authority histories.

Fixes #20735

Change-Id: I088136bc2e9d1b6bfdecb223c069d42400c9f63d
Signed-off-by: Michael Renner <terrorobe@github.com>