Summary:
Add the `ApplyToHandle` method to the `Cache` interface to allow a caller to request the invocation of a callback on the given cache handle. The goal here is to allow a cache that manages multiple cache instances to use a callback on a handle to determine which instance it belongs to. For example, the callback can hash the key and use that to pick the correct target instance. This is useful to redirect methods like `Ref` and `Release`, which don't know the cache key.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12987
Reviewed By: pdillinger
Differential Revision: D62151907
Pulled By: anand1976
fbshipit-source-id: e4ffbbb96eac9061d2ab0e7e1739eea5ebb1cd58
f48b64460e