[caching] Make LRUHandle and SlotHandle more type-safe.
[caching] Make LRUHandle and SlotHandle more type-safe.

It was possible to type-case a regular int to them eventually
causing panics or undefined behavior when calling LRU/Fetch.

Also reserve 0 value to mean "uninitialized". It means unregistered
handles are invalid (instead of pointing to first registered slot).

R=tandrii@chromium.org, 40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org

Change-Id: Idcdd6aca42d8661e71373f01295afaffd4ab7b3a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2242947
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go