Use NodeID2 in map hasher

[pav-kv]

MapHasher is the penultimate user of the NodeID type, which is being
removed soon. Since the maps are deprecated/experimental, we are free to
change the hasher interface to use a better NodeID2 type.

Part of #2378.

[codecov]

Codecov Report

Merging #2401 (592f4f6) into master (74e8305) will increase coverage by 0.03%.
The diff coverage is 61.90%.

@@            Coverage Diff             @@
##           master    #2401      +/-   ##
==========================================
+ Coverage   65.56%   65.60%   +0.03%     
==========================================
  Files         106      106              
  Lines        7702     7696       -6     
==========================================
- Hits         5050     5049       -1     
+ Misses       2120     2116       -4     
+ Partials      532      531       -1     

Impacted Files	Coverage Δ
merkle/maphasher/maphasher.go	71.87% <33.33%> (+3.12%)	⬆️
merkle/coniks/hasher/coniks.go	82.35% <69.23%> (+3.10%)	⬆️
experimental/batchmap/tmap.go	77.96% <100.00%> (-0.37%)	⬇️
merkle/smt/hasher.go	100.00% <100.00%> (ø)
log/operation_manager.go	88.35% <0.00%> (+1.05%)	⬆️

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[mhutchinson]

Cool. This does shine a light on github.com/google/trillian/storage/tree being a weird package; storage doesn't feel like something that should be on the API for clients to be leaning on.

My other concern with this is that this will need importing into trillian-examples and seeing what delta it causes for the maps there. However, that isn't going to happen until the pending update (google/trillian-examples#257) is applied, so I guess this will fall on me to maintain.

[pav-kv]

storage/tree needs decomposing into merkle (the core/algorithm bits), and storage-specific things. I will do this as part of this same refactoring. Previously some bits were moved here to avoid circular dependencies.