fix client reconnect offline provider.

[zonghaishang]

What is the purpose of the change

fix issue: #3158

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #3159 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3159      +/-   ##
============================================
- Coverage     63.78%   63.77%   -0.02%     
  Complexity       75       75              
============================================
  Files           661      661              
  Lines         28612    28612              
  Branches       4823     4823              
============================================
- Hits          18251    18246       -5     
+ Misses         8153     8151       -2     
- Partials       2208     2215       +7

Impacted Files	Coverage Δ	Complexity Δ
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	0% <0%> (ø)
...bo/rpc/cluster/support/FailbackClusterInvoker.java	67.21% <0%> (-3.28%)	0% <0%> (ø)
...pache/dubbo/remoting/transport/AbstractServer.java	44.79% <0%> (-3.13%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	64.93% <0%> (-1.74%)	0% <0%> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	73.64% <0%> (-1.36%)	0% <0%> (ø)
...pache/dubbo/registry/support/AbstractRegistry.java	79.61% <0%> (-1.16%)	0% <0%> (ø)
.../exchange/support/header/HeaderExchangeServer.java	60.37% <0%> (-0.95%)	0% <0%> (ø)
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	66.24% <0%> (+1.26%)	0% <0%> (ø)	⬇️
...he/dubbo/remoting/transport/netty/NettyServer.java	69.64% <0%> (+5.35%)	0% <0%> (ø)	⬇️
...che/dubbo/remoting/transport/mina/MinaChannel.java	53.94% <0%> (+10.52%)	0% <0%> (ø)	⬇️

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[CrazyHZM]

If there is any problem with the direct use reconnectExecutorFuture？
reconnectExecutorFuture .cancel(true)

[zonghaishang]

If there is any problem with the direct use reconnectExecutorFuture？
reconnectExecutorFuture .cancel(true)

@CrazyHZM This is defensive fault tolerance, calling reconnectExecutorFuture .cancel(true) will have no side effects.

[CrazyHZM]

If there is any problem with the direct use reconnectExecutorFuture？
reconnectExecutorFuture .cancel(true)

@CrazyHZM This is defensive fault tolerance, calling reconnectExecutorFuture .cancel(true) will have no side effects.

thank you.

[chickenlj]

LGTM

[zonghaishang]

merged: https://github.com/apache/incubator-dubbo/pull/3295/files