fix bugs related to purge rewrite

[tabokie]

~It's possible that certain raft groups are not compacted due to upper level issues. Those laggers will block the garbage collection progress indefinitely. After this patch, they will be rewritten regardless of the amount of their containing entries.~

Fix three bugs found during stability tests:

• Log entry hole between rewrite files crashed recovery
• Rewrite of a rewritten key could encounter a newer append and causes panic
• Rewrite log batch size exceeds i32::MAX and fails compression

[codecov[bot]]

Codecov Report

Merging #160 (0e1aed3) into master (a06397b) will decrease coverage by 0.06%. The diff coverage is 94.31%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
- Coverage   97.01%   96.94%   -0.07%     
==========================================
  Files          26       26              
  Lines        6523     6546      +23     
==========================================
+ Hits         6328     6346      +18     
- Misses        195      200       +5     

Impacted Files	Coverage Δ
src/engine.rs	92.97% <ø> (-0.05%)	:arrow_down:
src/purge.rs	96.09% <85.29%> (-1.40%)	:arrow_down:
src/memtable.rs	99.64% <100.00%> (+<0.01%)	:arrow_up:
src/metrics.rs	97.47% <100.00%> (+0.11%)	:arrow_up:
tests/failpoints/test_engine.rs	100.00% <100.00%> (ø)
src/codec.rs	98.34% <0.00%> (ø)

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