Add queryStream() method to stream result set rows and remove undocumented "results" event

[clue]

This PR adds a new queryStream() method which allows processing larger result sets with hundreds or thousands of records, so that one is no longer limited by memory in how many records can be processed. Its API can be used like this:

$stream = $connection->queryStream('SELECT * FROM users');

$stream->on('data', function ($row) {
    var_dump($row);
});
$stream->on('end', function () {
    echo 'DONE' . PHP_EOL;
});

Being a new addition to the ConnectionInterface, this PR is technically a BC break. This also includes a small patch to ensure that results are no longer buffered internally in the Parser and removes its undocumented results event. Empirical evidence suggests this should not affect most consumers.

Builds on top of #54
Resolves / closes #50