overhaul engine tests and fix bugs (#143)

Overhaul unit tests for engine. Discover and fix several bugs:

- First rewrite entry could have larger index than first append entry, and causes panic
- Region tombstone in append queue is not applied to rewrite queue during recovery
- [NOT FIXED] State tombstone is lost during rewrite #141
- Rewrite queue log index could jump back, and causes panic
- [NOT FIXED] Recreated region is deleted after rewrite #142

Signed-off-by: tabokie <xy.tao@outlook.com>