refactor: refactor propose id

[iGxnon]

Please briefly answer these questions:

proto changes: xline-kv/curp-proto#15 xline-kv/xline-proto#5

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

  Refactor propose id into ProposeId(u64, u64)

• 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

Attention: 57 lines in your changes are missing coverage. Please review.

Comparison is base (452c245) 54.12% compared to head (b5ca237) 54.25%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
+ Coverage   54.12%   54.25%   +0.13%     
==========================================
  Files         101      101              
  Lines       17856    17872      +16     
  Branches    17856    17872      +16     
==========================================
+ Hits         9664     9697      +33     
+ Misses       7615     7597      -18     
- Partials      577      578       +1     

Files	Coverage Δ
curp-test-utils/src/test_cmd.rs	77.81% <100.00%> (ø)
curp/src/log_entry.rs	72.41% <100.00%> (ø)
curp/src/rpc/connect.rs	24.73% <ø> (ø)
curp/src/server/cmd_worker/mod.rs	72.41% <100.00%> (ø)
curp/src/server/gc.rs	85.21% <100.00%> (ø)
curp/src/server/mod.rs	8.93% <100.00%> (ø)
curp/src/server/raw_curp/log.rs	88.44% <100.00%> (ø)
curp/src/server/raw_curp/tests.rs	73.93% <ø> (ø)
curp/src/server/spec_pool.rs	92.00% <100.00%> (ø)
xline/src/server/barriers.rs	77.17% <100.00%> (ø)
... and 9 more

... and 2 files with indirect coverage changes

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