[Dubbo-Optimization]: polish DubboBootstrap

[tangcent]

What is the purpose of the change

polish DubboBootstrap

Brief changelog

• use Collections.singletonList instead of Arrays.asList
• replace some lambda expressions to method reference to improve code readability.

Verifying this change

Run test cases.

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[codecov-io]

Codecov Report

Merging #5996 (1340dac) into master (279699f) will increase coverage by 0.03%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master    #5996      +/-   ##
============================================
+ Coverage     58.65%   58.68%   +0.03%     
  Complexity      424      424              
============================================
  Files          1043     1043              
  Lines         42473    42471       -2     
  Branches       6212     6212              
============================================
+ Hits          24912    24924      +12     
+ Misses        14773    14766       -7     
+ Partials       2788     2781       -7     

Impacted Files	Coverage Δ	Complexity Δ
.../apache/dubbo/config/bootstrap/DubboBootstrap.java	41.82% <40.00%> (+0.13%)	0.00 <0.00> (ø)
.../remoting/transport/netty4/NettyClientHandler.java	67.79% <0.00%> (-16.95%)	0.00% <0.00%> (ø%)
.../apache/dubbo/remoting/transport/AbstractPeer.java	58.69% <0.00%> (-4.35%)	0.00% <0.00%> (ø%)
...e/dubbo/remoting/exchange/codec/ExchangeCodec.java	74.56% <0.00%> (-0.88%)	0.00% <0.00%> (ø%)
...n/java/org/apache/dubbo/config/AbstractConfig.java	73.37% <0.00%> (+0.64%)	0.00% <0.00%> (ø%)
.../exchange/support/header/HeaderExchangeServer.java	66.98% <0.00%> (+1.88%)	0.00% <0.00%> (ø%)
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	88.57% <0.00%> (+1.90%)	16.00% <0.00%> (ø%)
...he/dubbo/common/threadpool/ThreadlessExecutor.java	96.15% <0.00%> (+1.92%)	0.00% <0.00%> (ø%)
...dubbo/remoting/exchange/support/DefaultFuture.java	88.03% <0.00%> (+2.56%)	0.00% <0.00%> (ø%)
...e/dubbo/remoting/transport/netty4/NettyServer.java	73.52% <0.00%> (+2.94%)	0.00% <0.00%> (ø%)
... and 5 more

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[AlbumenJ]

@tangcent hi, thanks for your contribution

pls resolve confilcts with the latest master branch

[AlbumenJ]

LGTM.