So that we can reliably ignore stale block services in GC (done in
a future commit). To enable this and future-proof this kind of
mechanism (e.g. having `eggsblocks` to mark something as D itself)
I added a new way to register the block service that lets you mask
which flags you're checking. I'll remove the old way once we've
rolled out everywhere.
The previous code was pretty nasty, it reached into the `ShardDB`
column family from another class. All those keys have been deleted
anyway in production.
This started being a problem since the block service update log
entry does not fit in a UDP packet (it's like 100KB). I think this
approach makes more sense anyway. See comment for `getCache()` for
gotchas.
