LLVMCAS: advanced size management

LLVMCAS OnDiskCAS implementation has a very simple size management, which doesn't allow deletion of the data entries and everything has to be removed all at once.

Simple size management is available via:
* Query an mostly accurate size estimate using APIs
* Chain two CAS databases to allow an old database fault into new database, so the old one can be deleted entirely after a while.

While the LLVMCAS library is designed to be simple and minimal function, and will often prefer performance over function, advanced size management like garbage collection or data expiration can be implemented if needed.