WIP: Enable operations that write to the DB to be rolled back on error by wrapping them in transactions.
WIP: Enable operations that write to the DB to be rolled back on error
by wrapping them in transactions.

Clean up: `delete` method of ValueManager does not need to return
anything, as it is always run from inside a Callback<Boolean>.

itsjeyd/ODE,itsjeyd/ODE,itsjeyd/ODE,itsjeyd/ODE