Streaming results leaks memory

[pabloromanelli]

AsyncSQLRowStream leeks memory because when making a query that returns a lot of rows, every row is stored inside the result set:

io.vertx.ext.asyncsql.impl.AsyncSQLRowStream

  AsyncSQLRowStream(QueryResult qr) {
    final Option<ResultSet> rows = qr.rows();
    if (rows.isDefined()) {
      rs = rows.get();
      cursor = rs.iterator();

com.github.mauricio.async.db.general.MutableResultSet

private val rows = new ArrayBuffer[RowData]()

And only when the query finishes, the result set is returned and the stream starts.
This behavior makes my application fail by OOM and can't handle streaming of large results.

This is not a feature currently supported by the client, but there is an open issue about this:
mauricio/postgresql-async#108

It would have been very useful for me to have a warning about this problem on the documentation before starting to code. Please consider adding it until fixed.