setRemoteTrafficLocked merged last_online with MAX(last_online, ?), which
is SQLite's two-argument scalar max. PostgreSQL's MAX() is aggregate-only,
so node traffic sync failed every cycle with "function max(bigint, unknown)
does not exist (SQLSTATE 42883)", flooding the logs.
Add a dialect-aware database.GreatestExpr helper (GREATEST on Postgres,
MAX on SQLite) and use it for the last_online merge. last_online is a
non-null int64, so the two functions are semantically identical here.
Closes#4633
8a28373a01