Feature/Xline client kv

[bsbds]

Implement kv client in xline-client

based on #317

Please briefly answer these questions:

• 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

Merging #318 (e17bd83) into master (c60a67d) will decrease coverage by 1.62%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   54.42%   52.81%   -1.62%     
==========================================
  Files          89       89              
  Lines       12822    13207     +385     
  Branches    12822    13207     +385     
==========================================
- Hits         6979     6975       -4     
- Misses       5392     5781     +389     
  Partials      451      451              

Impacted Files	Coverage Δ
xline-client/src/clients/kv.rs	0.00% <0.00%> (ø)
xline/src/server/auth_server.rs	0.00% <0.00%> (ø)
xline/src/server/kv_server.rs	39.26% <0.00%> (+0.10%)	⬆️
xline/src/server/lease_server.rs	0.00% <0.00%> (ø)
xline/src/server/lock_server.rs	0.00% <0.00%> (ø)
xlineapi/src/lib.rs	55.94% <0.00%> (+0.81%)	⬆️

... and 5 files with indirect coverage changes