[WIP] Output pos linked list

[antiochp]

Initial experiment with some primitives in store.rs toward a "linked list" maintained in lmdb for the output_pos index.

This is likely throwaway code. We still need to get the RFC together to decide on the final approach.

But I needed to sit down and write some code just to focus on something today for a couple of hours.

[antiochp]

Thinking about how we make this more generic, to support both output_pos and kernel_pos indexes. Presumably via a trait and associated type. Outputs will use OutputPos (with output features) and kernels the simpler CommitPos.

Or maybe just a LinkedList<OutputPos> and LinkedList<CommitPos> or something along those lines.

[antiochp]

The general approach here (once it works) feels like a good candidate for fully replacing #3228.
Rather than tracking "undo list" per block and rebuilding the index based on "undo" we track per kernel via the "linked list" pushing and popping off the stack of positions.

Proposed approach:

• Get this working here for kernel pos and cleanup
• rename branch and PR (for clarity)
• Think through the prune/compact logic -
  • during compaction we need to prune the kernel_pos index (keep 2 weeks of data)
• Implement for coinbase kernels initially (as proof of concept)?
  • We want to do this for NoRecentDuplicate kernels eventually but they do not yet exist

[antiochp]

Closing. Replaced with #3302.