refactor: modify --client-use-backoff commandline arg

[Phoenix500526]

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?) refactor the client-use-backoff cmd line arg.

• what changes does this pull request make?

  1. rename the client-use-backoff as client-fixed-backoff
  2. modify the type of client-fixed-backoff from Option to bool
  3. modify the default value of client-fixed-backoff

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

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d99ad0c) 74.71% compared to head (67d3e9d) 74.74%.

Files	Patch %	Lines
crates/xline/src/utils/args.rs	0.00%	2 Missing :warning:
crates/curp/src/client/mod.rs	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #651 +/- ## ========================================== + Coverage 74.71% 74.74% +0.02% ========================================== Files 167 167 Lines 24159 24159 Branches 24159 24159 ========================================== + Hits 18051 18058 +7 + Misses 4991 4982 -9 - Partials 1117 1119 +2 ```

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