Search index creation: Memory usage

Re-creating a search index takes up too much memory - here a graph during "user" index creation:
![screen shot 2014-02-12 at 21 17 23](https://f.cloud.github.com/assets/360800/2153116/191e1968-9423-11e3-8e2c-3af833e85152.png)

The code in question is in `CM_Elastica_Type_Abstract::update()`.
The graph suggests for ~5 minutes we're running the SQL SELECT query. Then when the results are coming in, the memory usage spikes immediately.

We're reading in rows one by one (`while ($row = $result->fetch()) {`).
Should investigate whether maybe the mysql result should be freed or something?

@kris-lab or @tomaszdurka please take over.