FlashCacheComponent using RegionManager's reclaim workers

[sumo-orange]

First of all, thanks for open-sourcing cachelib.

I was curious what the longer-term plan was for cachelib/interface/CacheComponent.h was. I was looking into writing a folly::coro wrapper for the navy block cache, but if CacheComponent and its impls are likely to remain in the repo, we'll probably try to use those.

I also have a specific question about how FlashCacheComponent is scheduling work: it looks like it's using the region manager's flush/reclaim worker NavyThread(s) to do the call into insert/find on the block cache (see FlashCacheComponent::onWorkerThread) -- is the long term plan to continue to do that?

Thanks!