Under-accounting of flush stats in case complex page iterators

Flush stats (fdSz and hdrSz) are under accounted in newPgOperator for complex pages with merge deltas. This discrepancy leads to inaccurate calculations(over-accounting) of FlushDataSz and FlushHdrSz in memory.

Consequently, LSS cleaners compute fragmentation incorrectly and run less frequently, causing accumulation of stale data on disk and resulting in disk bloat, particularly evident in workloads with frequent merges (e.g: timeseries data). A slow mutation rate aggravates the issue.
 
Workaround:
Recovery log blocks correctly persist flushDataSz and flushHdrSz without issues. During recovery, the in-memory stats FlushDataSz and FlushHdrSz are recomputed. Restarting the indexer process serves as a temporary fix.