Tracers: Determining UTxO set size in the UTxO HD era

[deepfire]

Since the UTxO is no longer entirely stored in memory, we're losing direct means of estimation of its size.

Yet, we still need this information, in the long run -- for performance estimation/prediction reasons.

[nfrisby]

Yeah, it should be easy enough to add a Word64 counter that gets included in some log Tracer events.

Edit: and I think double counting isn't a risk since that would correspond to double-spend and so would be a ledger failure.