Read-only transactions

[apkar]

It's just syntactic sugar on top of FDB transactions. This makes code much better and more readable by making it clear if a transaction was ever used for updates. We can't do this with current code, Flow bindings classes are not virtual.

blocked on: https://github.com/apple/foundationdb/issues/1027

[apkar]

Ideally, it should be in Flow bindings. But we can move it later.