* preserve the sum of certain tags (`step_count`, `parking:*:capacity`) during _join_ operation
* preserve total value of `parking:*:capacity` tags during _split_ operation by distributing it proportionally to the resulting ways
* the abstract osm entity now accepts a list of tags to override during the merging, but otherwise is agnostic about how tags can be merged: the concrete merging resolution might depend on the concrete action that was performed
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