[Forknet] Support split storage nodes in mirror mocknet test about nearcore HOT 1 OPEN

@marcelo-gonzalez I drafted a fix for starting split storage from genesis in this branch master...split-storage-on-genesis and I want you to test it in mocknet.

As it is a prod fix, I was focusing on a real-life scenario of starting split storage node from scratch.
So, for mocknet usage we still need to perform (some) extra steps. I tested this configuration as follows:

1. Created a node with big cold db
2. Pointed hot db (.store.path in config.json) to a new directory
3. Ran special tool to de-initialise cold db (while preserving data) neard cold-store -w make-cold-garbage (yes, we can technically change the name of this tool, but you will have to really convince me)
4. Ran neard neard run

As I didn't invest much effort into creating good testing environment, I couldn't test that both node and cold loop will work without problems when in sync.

For mocknet it shouldn't matter which db you attach as cold. These tools do not look at cold db past some keys in BlockMisc column. And those keys are deleted right away via neard cold-store -w make-cold-garbage.
So, I suggest

1. adding any cold disk to some mocknet nodes
2. modifying config on those nodes https://near-nodes.io/archival/split-storage-archival#config
3. running neard cold-store -w make-cold-garbage on them
4. starting mocknet test as usual (from genesis)

We then want to see those nodes fail to create a snapshot for resharding.
Any cold migration is a background process. During usual mocknet flow we will stop the node once it loaded genesis and make a copy of db for fast future restart. We need to add backup for cold db as well. It will probably not be initialised at that point, but the node from backup will still start ok. Background loop will just begin copying everything from hot to cold as a first step after a reset.

from nearcore.