Do DELETE instead of TRUNCATE when locks aren't acquired

After compression, the uncompressed part of the chunk is truncated.
This requires upgrading the `ExclusiveLock` to an
`AccessExclusiveLock` and hence is sometimes blocked by other other
operations, including reads, on the chunk. This leads to longer
compress times or potential deadlocks.

Instead of this, we fall back to deleting the tuples in the chunk
row-by-row when the upgraded lock isn't immediately acquired.

tsl/src/compression/compression.c

@@ -505,7 +505,15 @@ compress_chunk(Oid in_table, Oid out_table, int insert_options)
 	if (!ts_guc_enable_delete_after_compression)
 	{
 		DEBUG_WAITPOINT("compression_done_before_truncate_uncompressed");
-		truncate_relation(in_table);
+		if (ConditionalLockRelation(in_rel, AccessExclusiveLock))
+		{
+			truncate_relation(in_table);
+		}
+		else
+		{
+			/* Instead of waiting, delete rows one-by-one */
+			delete_relation_rows(in_table);
+		}
 		DEBUG_WAITPOINT("compression_done_after_truncate_uncompressed");
 	}
 	else

tsl/test/isolation/expected/compression_conflicts_iso.out

@@ -3023,7 +3023,12 @@ step CA1:
     CASE WHEN compress_chunk(ch) IS NOT NULL THEN true ELSE false END AS compress
   FROM show_chunks('ts_device_table') AS ch
   ORDER BY ch::text;
- <waiting ...>
+
+compress
+--------
+t       
+(1 row)
+
 step SA: SELECT * FROM ts_device_table;
 time|device|location|value
 ----+------+--------+-----
@@ -3041,10 +3046,4 @@ time|device|location|value
 
 step SF: 
 step SR: ROLLBACK;
-step CA1: <... completed>
-compress
---------
-t       
-(1 row)
-
 step CAc: COMMIT;