aristo: fork support via layers/txframes

[arnetheduck]

This change reorganises how the database is accessed: instead holding a "current frame" in the database object, a dag of frames is created based on the "base frame" held in AristoDbRef and all database access happens through this frame, which can be thought of as a consistent point-in-time snapshot of the database based on a particular fork of the chain.

In the code, "frame", "transaction" and "layer" is used to denote more or less the same thing: a dag of stacked changes backed by the on-disk database.

Although this is not a requirement, in practice each frame holds the change set of a single block - as such, the frame and its ancestors leading up to the on-disk state represents the state of the database after that block has been applied.

"committing" means merging the changes to its parent frame so that the difference between them is lost and only the cumulative changes remain - this facility enables frames to be combined arbitrarily wherever they are in the dag.

In particular, it becomes possible to consolidate a set of changes near the base of the dag and commit those to disk without having to re-do the in-memory frames built on top of them - this is useful for "flattening" a set of changes during a base update and sending those to storage without having to perform a block replay on top.

Looking at abstractions, a side effect of this change is that the KVT and Aristo are brought closer together by considering them to be part of the "same" atomic transaction set - the way the code gets organised, applying a block and saving it to the kvt happens in the same "logical" frame - therefore, discarding the frame discards both the aristo and kvt changes at the same time - likewise, they are persisted to disk together

• this makes reasoning about the database somewhat easier but has the downside of increased memory usage, something that perhaps will need addressing in the future.

Because the code reasons more strictly about frames and the state of the persisted database, it also makes it more visible where ForkedChain should be used and where it is still missing - in particular, frames represent a single branch of history while forkedchain manages multiple parallel forks - user-facing services such as the RPC should use the latter, ie until it has been finalized, a getBlock request should consider all forks and not just the blocks in the canonical head branch.

Another advantage of this approach is that AristoDbRef conceptually becomes more simple - removing its tracking of the "current" transaction stack simplifies reasoning about what can go wrong since this state now has to be passed around in the form of AristoTxRef - as such, many of the tests and facilities in the code that were dealing with "stack inconsistency" are now structurally prevented from happening. The test suite will need significant refactoring after this change.

Once this change has been merged, there are several follow-ups to do:

• there's no mechanism for keeping frames up to date as they get committed or rolled back - TODO
• naming is confused - many names for the same thing for legacy reason
• forkedchain support is still missing in lots of code
• clean up redundant logic based on previous designs - in particular the debug and introspection code no longer makes sense
• the way change sets are stored will probably need revisiting - because it's a stack of changes where each frame must be interrogated to find an on-disk value, with a base distance of 128 we'll at minimum have to perform 128 frame lookups for every database interaction - regardless, the "dag-like" nature will stay
• dispose and commit are poorly defined and perhaps redundant - in theory, one could simply let the GC collect abandoned frames etc, though it's likely an explicit mechanism will remain useful, so they stay for now

More about the changes:

• AristoDbRef gains a txRef field (todo: rename) that "more or less" corresponds to the old balancer field
• AristoDbRef.stack is gone - instead, there's a chain of AristoTxRef objects that hold their respective "layer" which has the actual changes
• No more reasoning about "top" and "stack" - instead, each AristoTxRef can be a "head" that "more or less" corresponds to the old single-history top notion and its stack
• level still represents "distance to base" - it's computed from the parent chain instead of being stored
• one has to be careful not to use frames where forkedchain was intended

• layers are only for a single branch of history!

TODO items for this PR:

• update forkedchain to use the new mechanism, solving Need to fix some misconception in ForkedChain #2949 in the process
• update / cleanup tests