optimize: do not load LoadBalance if not necessary

[wangliang181230]

Ⅰ. Describe what this PR did

optimize: Do not load LoadBalance if not necessary.
优化：当负载均衡不需要时，不加载它。

优化的原因：

部分用户的系统本来不大，单个seata服务就够用了。这时候跳过获取负载均衡器，性能也会好一些。
最主要的是，即使负载均衡的配置错误，也不会影响这部分用户的项目启动。

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-commenter]

Codecov Report

Merging #3735 (a6b78e6) into develop (b7943e9) will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@              Coverage Diff              @@
##             develop    #3735      +/-   ##
=============================================
- Coverage      51.33%   51.32%   -0.02%     
+ Complexity      3578     3575       -3     
=============================================
  Files            646      645       -1     
  Lines          21809    21808       -1     
  Branches        2737     2737              
=============================================
- Hits           11196    11192       -4     
- Misses          9477     9481       +4     
+ Partials        1136     1135       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ta/core/rpc/netty/AbstractNettyRemotingClient.java	17.96% <0.00%> (-0.45%)	8.00 <0.00> (ø)
...scovery/loadbalance/ConsistentHashLoadBalance.java	89.28% <100.00%> (ø)	3.00 <1.00> (ø)
.../discovery/loadbalance/LeastActiveLoadBalance.java	100.00% <100.00%> (ø)	7.00 <1.00> (ø)
...seata/discovery/loadbalance/RandomLoadBalance.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...a/discovery/loadbalance/RoundRobinLoadBalance.java	66.66% <100.00%> (ø)	3.00 <1.00> (ø)

[funky-eyes]

LGTM

[caohdgege]

LGTM