Fix bug and add test for dba_open same file twice

[chschneider]

When doing dba_open twice with the same file then dba_open tries to insert the same key into its hash table which triggers an assertion in a debug build and causes memory corruption in production.

A small reproduction script is

<?php
$a = dba_open("ext/dba/tests/test.cdb", "r");
$b = dba_open("ext/dba/tests/test.cdb", "r");
var_dump($a, $b);

The solution I chose is to add a counter to the keys generated with dba_open (but no dba_popen) to avoid key collisions.