Mention v20 upgrades can take a few minutes

[wezrule]

nanocurrency/nano-node#2283 is modifying all accounts to use the representative account instead of the representative block. This has been found to take a few minutes on some systems.

The account_info RPC should mark rep_block as deprecated and the representative shown by default when it eventually gets removed, RPC 2.0?

[zhyatt]

@wezrule What impacts to node operation are expected during this upgrade time? Is the node still expected to be participating on the network and responding to all RPC calls, etc.?

[wezrule]

The upgrade is synchronous and the node won't be participating or queryable during this. Even with the database upgrade for the epoch 2 preparation, the total downtime should be < 10 minutes

[wezrule]

Also mention that the ledger may grow in capacity by ~50%. A live ledger went from 16GB (before upgrade) to 24GB after. This is due to essentially copying all the state blocks to a new database while the old ones are still occupying space before being deleted. I experimented with using mdb_cursor_del to delete blocks while iterating over the old state v0/v1 databases, but it didn't make a difference to the size. --vacuum can be done after which gets it down to about ~13GB

[guilhermelawless]

We can note this in the migration page: https://docs.nano.org/releases/node-releases/#v200

[wezrule]

And done #156