* Simplify txFrame protocol, improve persist performance
To prepare forked-layers for further surgery to avoid the nesting tax,
the commit/rollback style of interacting must first be adjusted, since
it does not provide a point in time where the frame is "done" and goes
from being actively written to, to simply waiting to be persisted or
discarded.
A collateral benefit of this change is that the scheme removes some
complexity from the process by moving the "last saved block number" into
txframe along with the actual state changes thus reducing the risk that
they go "out of sync" and removing the "commit" consolidation
responsibility from ForkedChain.
* commit/rollback become checkpoint/dispose - since these are pure
in-memory constructs, there's less error handling and there's no real
"rollback" involved - dispose better implies that the instance cannot be
used and we can more aggressively clear the memory it uses
* simplified block number handling that moves to become part of txFrame
just like the data that the block number references
* avoid reparenting step by replacing the base instead of keeping a
singleton instance
* persist builds the set of changes from the bottom which helps avoid
moving changes in the top layers through each ancestor level of the
frame stack
* when using an in-memory database in tests, allow the instance to be
passed around to enable testing persist and reload logic
This kind of data is not used except in tests where it is used only to
create databases that don't match actual usage of aristo.
Removing simplifies future optimizations that can focus on processing
specific leaf types more efficiently.
A casualty of this removal is some test code as well as some proof
generation code that is unused - on the surface, it looks like it should
be possible to port both of these to the more specific data types -
doing so would ensure that a database written by one part of the
codebase can interact with the other - as it stands, there is confusion
on this point since using the proof generation code will result in a
database of a shape that is incompatible with the rest of eth1.
