I added the history mode configuration in eth/ethconfig initially, since
it seemed like the logical place. But it turns out we need access to the
intended pruning setting at a deeper level, and it actually needs to be
integrated with the blockchain startup procedure.
With this change applied, if a node previously had its history pruned,
and is subsequently restarted **without** the `--history.chain
postmerge` flag, the `BlockChain` initialization code will now verify
the freezer tail against the known pruning point of the predefined
network and will restore pruning status. Note that this logic is quite
restrictive, we allow non-zero tail only for known networks, and only
for the specific pruning point that is defined.
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.
