You could use something like async or just use a simple counter variable:
var conns = aArray.length;
aArray.forEach(function(u) {
pool.getConnection(function(err, connection) {
if (connection) {
var i = connection.query(aSQL, function(err, rows) {
if (--conns === 0)
pool.end();
});
} else if (--conns == 0)
pool.end();
connection.release();
});
});