Make it harder to mistake deletions for legitimate writes in commit hooks

[eddyashton]

Fixes #1208.

The value type in a Write is now std::optional<V> instead of VersionV<V>. The version was supposed to be NoVersion to indicate a deletion, but all of our own hooks were ignoring version and accessing value directly. assuming it was a real written value. This is a breaking API change - even if you want to ignore the possibility of deletions, you now need to access the actual values by .value() (preferably gated by a check to .has_value(),) not .value.

Our hooks don't expect deletions and don't have anything sensible to do if they do occur, so they all simply throw an exception. It would be nice to have this expectation statically enforced (some kind of NoRemovals modifier type for a kv::Map, which returned TxViews with no remove method, and whose Write type could store values directly), but this is an unnecessary extra bit of machinery for now.

[ghost]

distinct_deletes2@8746 aka 20200527.21 vs master ewma over 50 builds from 8546 to 8743