Feature/member add single

[themanforfree]

Please briefly answer these questions:
First PR of #306

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)
  make curp manage content other than Command.
• what changes does this pull request make?
  use Arc to share LogEntry, let conflict checked mpmc channel process LogEntry instead of Command
• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)
  no

[codecov]

Codecov Report

Patch coverage: 78.65% and project coverage change: -0.02% ⚠️

Comparison is base (9b747ec) 53.60% compared to head (405a02d) 53.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #382      +/-   ##
==========================================
- Coverage   53.60%   53.58%   -0.02%     
==========================================
  Files         102      102              
  Lines       15944    15976      +32     
  Branches    15944    15976      +32     
==========================================
+ Hits         8546     8561      +15     
- Misses       6878     6893      +15     
- Partials      520      522       +2     

Files Changed	Coverage Δ
curp/src/server/curp_node.rs	24.46% <0.00%> (ø)
curp/src/server/storage/mod.rs	0.00% <ø> (ø)
curp/src/server/storage/db.rs	67.64% <50.00%> (ø)
curp/src/log_entry.rs	52.38% <52.63%> (-22.62%)	⬇️
curp/src/server/raw_curp/mod.rs	66.14% <55.00%> (-0.58%)	⬇️
...urp/src/server/cmd_worker/conflict_checked_mpmc.rs	84.43% <87.17%> (-0.65%)	⬇️
curp/src/server/cmd_worker/mod.rs	70.81% <88.23%> (-1.53%)	⬇️
curp/src/server/raw_curp/log.rs	91.16% <96.96%> (+0.11%)	⬆️
curp/src/rpc/mod.rs	26.94% <100.00%> (ø)
curp/src/server/raw_curp/tests.rs	69.01% <100.00%> (ø)

... and 3 files with indirect coverage changes

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