refactor: build client without ServerId

[themanforfree]

Please briefly answer these questions:
#306

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)
  build client without ServerId
• what changes does this pull request make?
  add client builder
  add fetch_cluster rpc
  allows the client to build without ServerId, which automatically fetch the cluster information from the server
• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)
  yes, Client::new is removed, use client builder instead

[codecov]

Codecov Report

Patch coverage: 23.78% and project coverage change: +0.04% 🎉

Comparison is base (5126802) 53.69% compared to head (bad4986) 53.73%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
+ Coverage   53.69%   53.73%   +0.04%     
==========================================
  Files         101      101              
  Lines       15913    16074     +161     
  Branches    15913    16074     +161     
==========================================
+ Hits         8545     8638      +93     
- Misses       6847     6912      +65     
- Partials      521      524       +3     

Files Changed	Coverage Δ
benchmark/src/args.rs	0.00% <0.00%> (ø)
curp/src/rpc/mod.rs	25.12% <0.00%> (-1.83%)	⬇️
curp/src/server/curp_node.rs	23.94% <0.00%> (-0.52%)	⬇️
curp/src/server/mod.rs	0.00% <0.00%> (ø)
curp/src/server/raw_curp/mod.rs	65.86% <0.00%> (-0.86%)	⬇️
xline-client/src/clients/auth.rs	0.00% <ø> (ø)
xline-client/src/clients/kv.rs	0.00% <ø> (ø)
xline-client/src/clients/lease.rs	0.00% <ø> (ø)
xline-client/src/clients/lock.rs	0.00% <ø> (ø)
xline-client/src/clients/maintenance.rs	0.00% <ø> (ø)
... and 10 more

... and 6 files with indirect coverage changes

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