cacheserver: Add locking to the LRU The cache server will crash if two requests come in at the same time because the lack of locking. This change adds a mutex. I haven't seen the cache server crash since adding this. BUG=b:390255027 TEST=run cache server w/ foil-provision. Change-Id: Iaeb3c8d49d279c2a45faf45ffcb00b7666f7d9b3 Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/7013788 Auto-Submit: Raul Rangel <[email protected]> Reviewed-by: Seewai Fu <[email protected]> Commit-Queue: Raul Rangel <[email protected]> Tested-by: Raul Rangel <[email protected]>