Create StoreUpdate for one cold column (Block) #7744

[posvyatokum]

This is minimum working example of the first part of online copying
data from hot storage to cold.

The main function is StoreUpdate::add_cold_update. It needs to add
all the changes made to cold columns in one block to StoreUpdate. It
is done by finding all the relevant keys for every cold column and
inserting value for that key and column to update.

Because many columns have similar keys I introduce DBKeyType enum
which is meant to be semantical description of different key parts.
The main logical part related to DBKeyType is DBCol::key_type function
that is supposed to give a vector of key types that make a key for
a given column. And I to the best of my abilities define it for every
non-deprecated column. Although I really only need it for cold
columns. So, if nobody wants to check 50 key types I can define it
only for cold columns.

The point of get_keys_from_store() is to get all possible keys for
every DBKeyType needed for cold storage. It is intentionally only
defines only BlockHash right now, because the only cold column right
now is Block. It is ⋆｡°✩ minimal ✩‧₊˚ example.

copy_from_store() inserts update for given (col, key) into update.
I assume I wrote something dumb there, but it works, but I didn't give
it much thought.

Finally, I add simple cache to Store here, because during
get_keys_from_store we will read from a lot of cold columns that will
need to be later read again for inserting actual data into update.

I plan to add columns to DBCol::is_cold in next PRs without changing
this architecture much. The main change will happen in
get_keys_from_store.

[mina86]

This needs more documentation. Especially what Empty and Literal mean.

[posvyatokum]

I tried. Don't know if that helps.