Global order writer: initialize last_var_offsets_. (#2930)
Global order writer: initialize last_var_offsets_. (#2930)

Initializing last_var_offsets_ in the global order writer to prevent
unordered map threading issues.

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TYPE: IMPROVEMENT
DESC: Global order writer: initialize last_var_offsets_.
TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB