Here I am adding a config option and geth flag (`--history.chain`) for
configuring history pruning. There are two options available:
- `--history.chain all` is the default and will keep all history like
before.
- `--history.chain postmerge` will configure the history cutoff point to
the merge block.
The option doesn't actually do anything right now, but we need it as a
precursor for other history pruning changes.
