Session SQL databases survive server restarts

Databases created with the SQL database capability lived only in one server process’s memory, so a restart lost them and other server instances never saw them.

The server now keeps an in-memory SQLite cache per database and, after every write, stores only the changed 4 KB pages in PostgreSQL in one transaction. A server that does not have the database cached rebuilds it from the stored pages, and each commit is a compare-and-set so concurrent writers cannot overwrite each other. Agents keep using sql_execute, sql_query and sql_schema as before.

See SQL database and #4553.