optimize: add netty config constant keys.

[objcoding]

Ⅰ. Describe what this PR did

add netty config constant keys.

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 #1990 into develop will decrease coverage by <.01%.
The diff coverage is 66.66%.

@@              Coverage Diff              @@
##             develop    #1990      +/-   ##
=============================================
- Coverage      55.28%   55.27%   -0.01%     
+ Complexity      2432     2430       -2     
=============================================
  Files            432      432              
  Lines          14439    14439              
  Branches        1706     1706              
=============================================
- Hits            7982     7981       -1     
+ Misses          5735     5734       -1     
- Partials         722      724       +2

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/seata/core/constants/ConfigurationKeys.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...ava/io/seata/core/rpc/netty/NettyServerConfig.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ava/io/seata/core/rpc/netty/NettyClientConfig.java	45.31% <100%> (ø)	22 <3> (ø)	⬇️
.../java/io/seata/core/rpc/netty/NettyBaseConfig.java	36.11% <100%> (ø)	2 <0> (ø)	⬇️
...in/java/io/seata/server/session/GlobalSession.java	84.54% <0%> (-0.49%)	67% <0%> (-1%)
...o/seata/server/coordinator/DefaultCoordinator.java	48.01% <0%> (-0.4%)	27% <0%> (-1%)
...very/registry/zk/ZookeeperRegisterServiceImpl.java	63.7% <0%> (+0.8%)	24% <0%> (ø)	⬇️

[slievrly]

LGTM

[xingfudeshi]

LGTM.