Add cache for the TagFactory

[alexander-schranz]

Currently the TagFactory does always request the Tags here and as the query is based on "name" instead of "ids" doctrine will not use its internal cache for it:

https://github.com/sulu/SuluContentBundle/blob/0.4.0/Content/Infrastructure/Doctrine/TagFactory.php#L45-L50

For performance reasons e.g. imports this is not very good maybe we can introduce simple memory based cache e.g.:

// after query result
foreach ($result as $tag) {
    $this->tags[$tag->getName()] = $tag;
}

// after new tag created also add it to the array
$this->tags[$tag->getName()] = $tag;

So we can only query the not yet found tags.

Open Question:

• Need this cache be invalidated?
  • Edge Case in the same process a Tag is removed
  • EntityManager is cleared (which happens mostly in imports) to clear memory (alternative cache just "id" of the tag instead of the whole object this would keep the memory very low)