add option for Tier0 specific LFNs

Tier0 LFNs end with three directory levels based on the run number
Move DB imports out of module-level

DAOFactory gets imported by different CRAB bits, but that's bad if
the three dialects aren't loaded in SQLAlchemy. Push this inside the
module
Merge pull request #4858 from PerilousApricot/bugfix/realign-imports

Move DB imports out of module-level
Updating oneEventMode for WMCore

Make the rev-limiter configurable in the step
Merge pull request #4859 from PerilousApricot/feature/oneEventMode

Updating oneEventMode for WMCore
Merge pull request #4861 from hufnagel/tier0-lfn

add option for Tier0 specific LFNs