xtd-files-fragmentation-optimization

Currently (as of pbxt 1.0) free space in .xtd files is accounted using a list of free record slots and every new record will occupy the first available slot. This can lead to high fragmentation. To avoid fragmentation the following approaches can be used:

1. use free-space allocation algorithms that minimize fragmentation. E.g. use free space maps or b-trees of free space extents, utilize other algorithms used by filesystems, rely more on record clustering;

2. use d-log type allocation where new records only appended to the end, always sequentially and free space is optimized in background;

to test the efficiency of the second method we can use AVG_ROW_LENGTH table creation option to move more data int d-logs.