Reduce code complexity related to maps

[pav-kv]

Some of the code is unnecessarily complex because previously it tried to accommodate both Log and Map trees. After maps removal [#2284] it makes sense to minimize this code, so that it's log-specific. If anything related to maps reappears in the repo, it still makes sense to keep the code decoupled to a greater degree than previously.

Examples:

• [ ] storage/cache can be removed completely, or significantly simplified.
• [x] The NodeID&co types should be made log-specific. It likely needs to be replaced by a simple compact.NodeID, whereas for maps the NodeID2 type is more suitable.
• [ ] The LogStorage/TreeStorage/MapStorage interfaces can be reduced.

[mhutchinson]

@pavelkalinnikov is the checklist in the original comment still accurately tracking the remaining work on this?

[pav-kv]

Yes, it still looks relevant.