Separate TBM[Shared|Private]Iterator and TBMIterateResult

Remove the TBMIterateResult member from the TBMPrivateIterator and
TBMSharedIterator and make tbm_[shared|private_]iterate() take a
TBMIterateResult as a parameter.

This allows tidbitmap API users to manage multiple TBMIterateResults per
scan. This is required for bitmap heap scan to use the read stream API,
with which there may be multiple I/Os in flight at once, each one with a
TBMIterateResult.

Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/d4bb26c9-fe07-439e-ac53-c0e244387e01%40vondra.me

src/include/access/gin_private.h

@@ -354,7 +354,12 @@ typedef struct GinScanEntryData
 	/* for a partial-match or full-scan query, we accumulate all TIDs here */
 	TIDBitmap  *matchBitmap;
 	TBMPrivateIterator *matchIterator;
-	TBMIterateResult *matchResult;
+
+	/*
+	 * If blockno is InvalidBlockNumber, all of the other fields in the
+	 * matchResult are meaningless.
+	 */
+	TBMIterateResult matchResult;
 	OffsetNumber matchOffsets[TBM_MAX_TUPLES_PER_PAGE];
 	int			matchNtuples;