Remove PENDING_TASKS.

[carryxyh]

Remove PENDING_TASKS.

Refactor for:
#4040

Related for:
#4085

[codecov-io]

Codecov Report

Merging #4087 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master   #4087      +/-   ##
===========================================
+ Coverage     62.69%   62.7%   +0.01%     
  Complexity      544     544              
===========================================
  Files           762     762              
  Lines         32758   32757       -1     
  Branches       5165    5165              
===========================================
+ Hits          20538   20541       +3     
+ Misses         9861    9857       -4     
  Partials       2359    2359

Impacted Files	Coverage Δ	Complexity Δ
...dubbo/remoting/exchange/support/DefaultFuture.java	75% <100%> (-0.17%)	0 <0> (ø)
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	0% <0%> (ø)
.../apache/dubbo/remoting/transport/AbstractPeer.java	63.04% <0%> (ø)	0% <0%> (ø)	⬇️
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	80.88% <0%> (+0.73%)	0% <0%> (ø)	⬇️
...he/dubbo/registry/multicast/MulticastRegistry.java	69.68% <0%> (+1.8%)	0% <0%> (ø)	⬇️
...bo/rpc/cluster/support/FailbackClusterInvoker.java	70.49% <0%> (+3.27%)	0% <0%> (ø)	⬇️
.../apache/dubbo/qos/protocol/QosProtocolWrapper.java	69.23% <0%> (+5.12%)	0% <0%> (ø)	⬇️

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