Macro db_client 

macro_rules! db_client {
    ($marker:ident, $name:literal, max = $max:expr) => { ... };
    ($marker:ident, $name:literal, max = $max:expr, statement_timeout_secs = $secs:expr) => { ... };
    (@build $marker:ident, $name:literal, $max:expr, $timeout:expr) => { ... };
}

Declare a database client: a named pool with its own size and optional statement timeout.