Cache query plans

[mwest1066]

Change sql-loader.load() to return an object rather than a string, including a unique name:

{
    name: filename + ‘-‘ + block_name,
    text: sql_string
}

See https://node-postgres.com/api/client#-code-client-query-config-queryconfig-gt-promise-lt-result-gt-code-

This triggers execution plan caching in the DB: https://node-postgres.com/features/queries#parameterized-query