callback_on_evict doesn't provide value_size

[kilobyte]

I expect that most of the time an user writes an on_evict callback, it's to do something to the entry that's being evicted. That usually requires knowing the value's size.

With our current API, this requires two gets, going through the index and repl twice. That's a waste of time — we already know value_size.

It'd be nice to pass it to the callback. Alas, that'd be an API break...

I'm thus saving this as a note we may want to consider in case we'd want to break the API before 1.0. Another option would be to have a callback2, but that's ugly.