The key idea here is that even though we can't ensure MPT code won't make the
node active again we can order the changes made to the persistent store in
such a way that it practically doesn't matter. What happens is:
* after persist if it's time to collect our garbage we do it synchronously
right in the same thread working the underlying persistent store directly
* all the other node code doesn't see much of it, it works with bc.dao or
layers above it
* if MPT doesn't find some stale deactivated node in the storage it's OK,
it'll recreate it in bc.dao
* if MPT finds it and activates it, it's OK too, bc.dao will store it
* while GC is being performed nothing else changes the persistent store
* all subsequent bc.dao persists only happen after the GC is completed which
means that any changes to the (potentially) deleted nodes have a priority,
it's OK for GC to delete something that'll be recreated with the next
persist cycle
Otherwise it's a simple scheme with node status/last active height stored in
the value. Preliminary tests show that it works ~18% worse than the simple
KeepOnlyLatest scheme, but this seems to be the best result so far.
Fixes#2095.
423c7883b8