refactor: request validation check

[bsbds]

Described in: #398
Based on: #399

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?)

  Currently some of the validation checks are done in the RPC server. However, these checks are not applied in the storage request handlers (eg. handle_range_request), so that the request from outside client will not be checked.

• what changes does this pull request make?

  This pull request applied the validation check in both RPC server and the storage handlers.

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Patch coverage: 82.44% and project coverage change: +1.01% 🎉

Comparison is base (6f1cff4) 52.44% compared to head (38eab5e) 53.46%.
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   52.44%   53.46%   +1.01%     
==========================================
  Files          98      100       +2     
  Lines       15100    15531     +431     
  Branches    15100    15531     +431     
==========================================
+ Hits         7919     8303     +384     
- Misses       6686     6727      +41     
- Partials      495      501       +6     

Files Changed	Coverage Δ
xline/src/lib.rs	100.00% <ø> (ø)
xline/src/server/auth_server.rs	0.00% <0.00%> (ø)
xline/src/storage/auth_store/store.rs	56.75% <0.00%> (-0.16%)	⬇️
xline/src/storage/kv_store.rs	71.69% <65.38%> (+0.68%)	⬆️
xline/src/server/kv_server.rs	41.77% <81.08%> (-10.52%)	⬇️
xline/src/request_validation.rs	84.41% <84.41%> (ø)
xline/src/revision_check.rs	91.66% <91.66%> (ø)
xline/src/storage/execute_error.rs	35.71% <100.00%> (+35.71%)	⬆️

... and 8 files with indirect coverage changes

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