FIX: Temporary Directory for Catalog Migrations

The catalog migrations were performed in `/srv/cvmfs/.../data/txn` which might not be local storage in production systems. This potentially slows down the catalog migration since SQLite would do the database mangling remotely. (For example on the CERN stratum zeros this would happen over NFS)

See [CVM-656](https://sft.its.cern.ch/jira/browse/CVM-656).