Add support for changing the charge for a cache entry

The Cache implementation in be/src/util/cache currently does not support modifying the charge of a cache entry after it has been created. For cases where the size is known up front, this is fine. For example, the data cache knows the number of bytes it will consume before it creates the cache entry.

This is a problem for caches that may not know the size of an entry up front. For example, the tuple cache may want to create a cache entry immediately to avoid concurrency issues, but then it would want to update that entry's charge as the entry is finalized (or reaches certain size increments).

It would also be useful to expose the maximum charge allowed for a cache entry. This would allow writers to avoid creating a cache entry that is too large.