Optimization of Catalog Handling During Publish

This should speed up the catalog handling (creation, committing) substantially. The problem was, that both initial fill on catalog creation and meta data updates on catalog commit were not processed inside an SQLite transaction.

On my dev-machine the following script runs about 3 to 4 times faster after this change:
```
#!/bin/bash
repo=test.local
cvmfs_server mkfs -o root $repo
cvmfs_server transaction $repo
mkdir /cvmfs/$repo/{a,b,c,d,e,f,g,h,i,j,k,l,m}
touch /cvmfs/$repo/{a,b,c,d,e,f,g,h,i,j,k,l,m}/.cvmfscatalog
cvmfs_server publish -v $repo
cvmfs_server rmfs -f $repo
```