Fix segfault in PixelTrackReconstruction

Fixes the segfault reported by Giulio in https://hypernews.cern.ch/HyperNews/CMS/get/swReleases/4012.html. The problem was a spurious delete of ```theRegionProducer``` in ```halt()``` (called from ```PixelTrackProducer::endRun()```), while it is constructed now in the PixelTrackReconstruction constructor. The bug was introduced in 4f569be1 (probably by mistake in rebase or something).

The ```theMerger_``` would have suffered from the same problem, so I changed it too to ```std:unique_ptr```.