AVM: Add an unlimited global storage mechanism - Boxes.

[jannotti]

This is the initial check-in to feature branch. I'll ask for reviews now, to shake out issues that I have not considered, but it will remain a WIP for a while longer, so final details are not important. We want to have a feature branch that other team members can work on.

Some things to think about/finish

• It has no LRU cache for the reads and writes. I expect sqlite to
  do a fine job caching kv pairs, and there's not savings from skipping
  msgpack serialization. But sqlite has a page cache, not a row cache,
  so maybe it's worth it.
• There is no loop around the sql read with round number test.
• There's is no locking of au.accountsMu. This is clearly wrong,
  need to learn more about the rules that govern when the other lookup
  routines are not taking lock.
• Need to think through limits on total boxes (and bytes) read/written
• Catchpoints
• transaction.WellFormed should check box indexes
• ops costs, especially for box_create and box_del, need to be thought out
• consider putting in new rules for hash costs

[michaeldiamant]

Let's bring the functionality into the feature branch for easier downstream consumption. 👍

[zoultrex]

This is really awesome, is there any ETA on when this is coming out?

[jannotti]

This is really awesome, is there any ETA on when this is coming out?

Keep your eyes on https://github.com/algorand/go-algorand/tree/feature/avm-box

Probably August.