Implement a new Tier-0 mode

This one deletes blocks from T2_CH_CERN after they have been processed and have
been completely transferred to a custodial location. If a dataset subscription
exists for T2_CH_CERN then all blocks in that dataset are not deleted.

Since the block check can't know which blocks have been already deleted, it
will search for all blocks in the DBSBuffer, this is limited by creation date of the block
which defaults to two-day old blocks. Configurable through config.PhEDExInjector.blockTimeout