refactor FilePipeLog and improve latency metering

[tabokie]

Improve latency metering, in specific, add duration metrics for writer wait, log rotate, log allocate. Add counter metrics for entry count. Refactor FilePipeLog into a separate module, and move recovery logic into builder.

Fix bugs:

• Data inconsistency after recovery when there is hole between files
• Statistics is split into two versions after recovering rewrite queue
• Garbage ratio statistics is inaccurate and might cause frequent rewrite

[codecov[bot]]

Codecov Report

Merging #153 (0a12d02) into master (ce65f11) will increase coverage by 0.14%. The diff coverage is 97.13%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   96.73%   96.87%   +0.14%     
==========================================
  Files          23       25       +2     
  Lines        6086     6176      +90     
==========================================
+ Hits         5887     5983      +96     
+ Misses        199      193       -6     

Impacted Files	Coverage Δ
src/file_pipe_log/log_file.rs	92.15% <ø> (ø)
src/pipe_log.rs	93.10% <ø> (ø)
src/file_pipe_log/format.rs	90.52% <90.52%> (ø)
src/file_pipe_log/builder.rs	96.04% <96.04%> (ø)
src/file_pipe_log/pipe_log.rs	97.29% <97.29%> (ø)
src/memtable.rs	99.44% <99.03%> (-0.06%)	:arrow_down:
src/engine.rs	94.83% <100.00%> (+0.07%)	:arrow_up:
src/file_pipe_log/reader.rs	88.00% <100.00%> (ø)
src/lib.rs	100.00% <100.00%> (ø)
src/metrics.rs	97.36% <100.00%> (+0.70%)	:arrow_up:
... and 4 more

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