Refactor `influxd inspect export-lp` to have more predictable resource usage

[danxmoran]

The 2.x implementation of export-lp introduced in #20467 uses the ReadAll method on TSMReader to get the field-values for each exported series key. This matches the 1.x implementation, but was flagged during review as a lurking OOM for series keys with many fields.

Instead of ReadAll, we could use the BlockIterator method on TSMReader to process one block of data at a time. The new processing logic might need to copy some code out of ReadAll to ensure we're handling tombstones etc. in the same way.