DomainSensitiveFrontier is slow and does not supply the wanted functionality

The DomainSensitiveFrontier has turned out to be slow on crawls with lots of
seeds (e.g. 10.000)
Further more is the max-docs setting applied on a pr. host basis instead of a
pr. domain basis.
Suggest we shift to the default BdbFrontier
NOTE: This bug is originally from Bugzilla bug_id=395.