2025 DatabaseAccess overhaul #4

We're in 2025 and we should restructure the database access for the future:

• remove non-async interfaces from DatabaseAccessAbstractions, eliminate Async prefix in remaining interfaces (I/O with third-party systems should always be async)
• rename IReadOnlySession to IClient - even if callers do not want to explicitly call SaveChangesAsync, their might be data manipulation going on. This kind of humble object simply behaves like an HttpClient - there could be a transaction, there might be immediate data manipulation, we simply don't know that it is read-only. We will drop the IReadOnlySession interface altogether and simply rely on IAsyncDisposable.
• Rename the namespace DatabaseAccessAbstractions to DataAccessAbstractions - when we have an IClient, we cannot be sure that we connect to a database.
• Drop the interfaces IAsyncTransactionalSession and IAsyncTransaction.