Optimize `purge_expired_files`

[tabokie]

Several optimizations for purge_expired_files:

• Support configuring purge-rewrite-threshold and purge-rewrite-garbage-ratio
• Tune up garbage ratio to 0.6
• Tune down rewrite log batch size to 120KB, reducing the compression overhead when rewriting rewrite queue
• Purge immediately after rewriting append queue

Also fix a statistics bug.

[codecov[bot]]

Codecov Report

Merging #162 (9d9daed) into master (141d2e5) will decrease coverage by 0.00%. The diff coverage is 98.48%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   96.97%   96.96%   -0.01%     
==========================================
  Files          26       26              
  Lines        6582     6597      +15     
==========================================
+ Hits         6383     6397      +14     
- Misses        199      200       +1     

Impacted Files	Coverage Δ
src/purge.rs	96.71% <97.67%> (-0.33%)	:arrow_down:
src/config.rs	94.00% <100.00%> (+0.89%)	:arrow_up:
src/memtable.rs	99.50% <100.00%> (+<0.01%)	:arrow_up:
src/metrics.rs	97.50% <100.00%> (+0.02%)	:arrow_up:

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