Add feed index for batch publish

[Riateche]

• Store last assigned feed index in the permission account.
• Assign new feed index when creating a new price feed.
• Add a migration instruction to assign feed index to an existing feed.

reserve_new_price_feed_index gets called from init_price_feed_index (to assign an index to an existing price) and from add_price (to assign an index to a new price).
The transition process would be to top up the permission account so that it's rent exempt with the new size, then to call init_price_feed_index for every existing price feed.

[ali-bahjati]

Looks great! I'll approve once we have some tests for it too.

[guibescos]

I think MIN_SIZE_WITH_LAST_FEED_INDEX, INITIAL_SIZE and NEW_ACCOUNT_SPACE are all the same, can't we use INITIAL_SIZE everywhere?

[Riateche]

I think MIN_SIZE_WITH_LAST_FEED_INDEX, INITIAL_SIZE and NEW_ACCOUNT_SPACE are all the same, can't we use INITIAL_SIZE everywhere?

Originally, I tried to alter INITIAL_SIZE for PermissionAccount, but I realized that unfortunately INITIAL_SIZE is not the initial size of the account, but the initial size of the initialized data. The naming is a bit counter-intuitive. INITIAL_SIZE can in fact be smaller than MINIMUM_SIZE for accounts that have variable data length. So originally MINIMUM_SIZE was used both as the space to create the account and as the required minimum size when reading the account. Now that I want to support reading permission accounts with old (smaller) size but at the same time I want to create new permission accounts with new (larger) size, I have to introduce a separate constant (NEW_ACCOUNT_SPACE) to distinguish between these use cases specifically for the permission account.

I've removed MIN_SIZE_WITH_LAST_FEED_INDEX because it's really the same thing as NEW_ACCOUNT_SPACE now.

[ali-bahjati]

Nice!