overhaul engine tests and fix bugs

[tabokie]

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 Key value tombstone is lost during rewrite #141
• Rewrite queue log index could jump back, and causes panic
• [NOT FIXED] Recreated region is deleted after rewrite Raft Group tombstone is rewritten in the wrong order #142

Signed-off-by: tabokie [email protected]

[codecov]

Codecov Report

Merging #143 (2b67ea1) into master (7c2c239) will increase coverage by 0.67%.
The diff coverage is 98.44%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   96.06%   96.73%   +0.67%     
==========================================
  Files          22       22              
  Lines        5486     5786     +300     
==========================================
+ Hits         5270     5597     +327     
+ Misses        216      189      -27     

Impacted Files	Coverage Δ
src/file_builder.rs	100.00% <ø> (ø)
src/test_util.rs	95.00% <85.00%> (-5.00%)	⬇️
src/engine.rs	96.33% <98.36%> (+3.11%)	⬆️
src/file_pipe_log.rs	95.45% <100.00%> (+0.42%)	⬆️
src/log_batch.rs	98.37% <100.00%> (+0.92%)	⬆️
src/memtable.rs	99.49% <100.00%> (+0.73%)	⬆️
src/purge.rs	94.51% <100.00%> (+0.57%)	⬆️
... and 2 more

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