Make the query builder immutable

[baukevdw]

Making the query builder immutable makes code safer and allows the user to write cleaner code.

<?php
/** @var \Neat\Database\Connection $db */
$query = new \Neat\Database\Query($db);

$usersQuery         = $query->from('user', 'u');
$totalUsers         = $usersQuery->select('COUNT(1)')->query()->value();
$users              = $usersQuery->select('u.*')->limit(5)->query()->rows();
$activeUsersQuery   = $usersQuery->where(['active' => true]);
$totalActiveUsers   = $activeUsersQuery->select('COUNT(1)')->query()->value();
$activeRoles        = $activeUsersQuery->select('DISTINCT u.role')->query()->values();
$activeUsers        = $activeUsersQuery->select('u.*')->limit(5)->query()->rows();

Making the query builder immutable might break code so we'll need an upgrade path:
1.x release:

• Introduce QueryBuilderInterface
• Introduce ImmutableQuery implements QueryBuilderInterface
• Introduce MutableQuery extends Query implements QueryBuilderInterface
• Deprecate Query

2.0 release:

• Query will be immutable
• ImmutableQuery will extend Query