Make action cache concurrency-friendly to avoid high contention. Use...

Make action cache concurrency-friendly to avoid high contention. Use ConcurrentHashMap so there's no need for locking. Keep entries to be written to disk in a concurrent queue, and look their values up in the canonical map when writing them. This avoids races around writing data to the maps, although I don't know how big a deal that is anyway.

Doesn't seem to have much of a wall-time impact, most likely because contention just moves around. Hoping to tackle some of the ultimate causes soon.

PiperOrigin-RevId: 389272785