Feature/auto compactor

[Phoenix500526]

Please briefly answer these questions:
based on pr #399 and #394

1. implement auto-compaction logic, read [Proposal] Compaction Feature Designed Doc #188 for more details
2. break down the dependencies between CurpServer and CurpClient.

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Patch coverage: 61.05% and project coverage change: +0.12% 🎉

Comparison is base (b4ef3c8) 53.48% compared to head (39478eb) 53.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   53.48%   53.61%   +0.12%     
==========================================
  Files         100      102       +2     
  Lines       15531    15944     +413     
  Branches    15531    15944     +413     
==========================================
+ Hits         8307     8548     +241     
- Misses       6725     6877     +152     
- Partials      499      519      +20     

Files Changed	Coverage Δ
curp/src/log_entry.rs	75.00% <ø> (ø)
curp/src/rpc/mod.rs	26.94% <ø> (ø)
curp/src/server/cmd_worker/mod.rs	72.34% <ø> (ø)
curp/src/server/curp_node.rs	24.46% <ø> (+0.15%)	⬆️
curp/src/server/gc.rs	86.84% <ø> (ø)
curp/src/server/mod.rs	0.00% <ø> (ø)
curp/src/server/raw_curp/log.rs	91.04% <ø> (ø)
curp/src/server/raw_curp/mod.rs	66.57% <ø> (+0.61%)	⬆️
curp/src/server/raw_curp/tests.rs	69.01% <ø> (ø)
curp/src/snapshot.rs	87.50% <ø> (ø)
... and 16 more

... and 1 file with indirect coverage changes

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