test: Check that stats are not recomputed

[bitdivine]

Motivation

Until the least release, account stats such as the total number of hardware wallets was computed as part of every upgrade in the post_upgrade hook. Iterating over all accounts on the heap was acceptably fast. This is absolutely not the case when accounts are stored in stable memory. Using the same code path when stable memory is in use would cause the upgrade to fail. We therefore wish to:

• Test that stats are not recomputed on upgrade any more, during normal clean upgrades.
• Delete the code for recomputing stats once we are sure that the new state of affairs is stable.

Changes

• Make it visible whether stats were recomputed during an upgrade by recording this in the public stats.
• Add a test that, with normal downgrades and upgrades, stats are never recomputed.

Tests

• A test is included.

Todos

• Add entry to changelog (if necessary).