The pending_block member of database was a premature optimization and had an
unfortunate tendency to get out of sync, especially when switching forks.
This commit removes it, and substantially improves the handling of transactions
when switching forks. Specifically, flooding or forking no longer causes nodes to
discard valid transactions.
- with this change the fork database now caches blocks that do not link
and attempts to link the unlinked blocks after every new block is
successfully linked.
- with this change the fork database only tracks blocks as far back as
the undo history allows. This significantly reduces memory usage when
there is high witness participation.
