change the key type to String

[nikyotensai]

Ⅰ. Describe what this PR did

The return type of ctx.channel().remoteAddress() isn't String.I changed it.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #923 into develop will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             develop    #923      +/-   ##
============================================
- Coverage      38.51%   38.5%   -0.01%     
  Complexity      1035    1035              
============================================
  Files            222     222              
  Lines           8662    8664       +2     
  Branches        1048    1048              
============================================
  Hits            3336    3336              
- Misses          4933    4935       +2     
  Partials         393     393

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/seata/core/rpc/netty/RmRpcClient.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...main/java/io/seata/core/rpc/netty/TmRpcClient.java	29.16% <0%> (-0.18%)	12 <0> (ø)

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[CoffeeLatte007]

LGTM

[slievrly]

LGTM