Client discover cluster

[themanforfree]

Please briefly answer these questions:
described in #306
based on #449

• 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

Attention: 178 lines in your changes are missing coverage. Please review.

Comparison is base (5cee011) 54.03% compared to head (de4f2c8) 53.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
- Coverage   54.03%   53.73%   -0.30%     
==========================================
  Files         101      101              
  Lines       17918    18093     +175     
  Branches    17918    18093     +175     
==========================================
+ Hits         9682     9723      +41     
- Misses       7659     7793     +134     
  Partials      577      577              

Files	Coverage Δ
curp/src/error.rs	8.57% <ø> (ø)
curp/src/server/raw_curp/tests.rs	73.93% <ø> (ø)
curp/src/log_entry.rs	80.76% <0.00%> (+8.35%)	⬆️
...urp/src/server/cmd_worker/conflict_checked_mpmc.rs	84.49% <0.00%> (ø)
curp/src/server/cmd_worker/mod.rs	72.41% <0.00%> (ø)
xline/src/server/cluster_server.rs	0.00% <0.00%> (ø)
xline-client/src/error.rs	0.00% <0.00%> (ø)
curp/src/server/raw_curp/mod.rs	67.56% <30.43%> (+0.04%)	⬆️
curp/src/members.rs	76.81% <34.48%> (-4.86%)	⬇️
curp/src/rpc/mod.rs	25.56% <8.00%> (-1.24%)	⬇️
... and 2 more

... and 3 files with indirect coverage changes

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