fix searching by node name by only storing the namespace alias.

previously we stored the entire URL, without this change the search tests fails to find any nodes.
i guess this is what the following code was trying to deal with:
https://github.com/jackalope/jackalope-doctrine-dbal/pull/134/files#L0L584

the issue is caused by:
https://github.com/jackalope/jackalope-doctrine-dbal/blob/master/src/Jackalope/Transport/DoctrineDBAL/Query/QOMWalker.php#L507

which results in the following to be added to the query:
```
n0.namespace || (
      CASE n0.namespace WHEN '' THEN '' ELSE ':' END
    ) || n0.local_name = 'phpcr_locale:en'
```

i guess with this change we would then somehow need to expand to an URL when we read back out of the DB? furthermore do we then risk some inconsistencies? i guess the alias needs to be unique?

/cc @dbu @wjzijderveld