add new thread pool strategy for Consumer, allow all service calls to share the same thread pool.

[chickenlj]

If you want to share one single thread pool for all services and requests, configure like this:

<dubbo:consumer threadpool="fixed" threads="700">
        <dubbo:parameter key="share-threadpool" value="true"/>
</dubbo:consumer>

There's no strict rules for which strategy is the best, every strategy has its pros and cons, just pick the one that is suitable for your case. Sharing the same fixed-size thread pool can be very helpful for Consumers working as a proxy.

[codecov-io]

Codecov Report

Merging #5891 into 2.6.x will decrease coverage by 0.08%.
The diff coverage is 72.72%.

@@             Coverage Diff              @@
##              2.6.x    #5891      +/-   ##
============================================
- Coverage     47.47%   47.38%   -0.09%     
+ Complexity     4609     4608       -1     
============================================
  Files           577      577              
  Lines         26509    26455      -54     
  Branches       4691     4694       +3     
============================================
- Hits          12585    12536      -49     
+ Misses        12007    12001       -6     
- Partials       1917     1918       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com/alibaba/dubbo/common/Constants.java	88.88% <ø> (ø)	1.00 <0.00> (ø)
...ibaba/dubbo/remoting/transport/AbstractClient.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ba/dubbo/common/store/support/SimpleDataStore.java	94.44% <100.00%> (ø)	8.00 <1.00> (ø)
...ng/transport/dispatcher/WrappedChannelHandler.java	55.31% <100.00%> (+20.18%)	8.00 <4.00> (+4.00)
...alibaba/dubbo/rpc/protocol/thrift/ThriftUtils.java	63.15% <0.00%> (-12.26%)	6.00% <0.00%> (ø%)
...aba/dubbo/remoting/transport/mina/MinaChannel.java	42.25% <0.00%> (-11.27%)	15.00% <0.00%> (-1.00%)
.../remoting/transport/netty4/NettyServerHandler.java	72.22% <0.00%> (-11.12%)	6.00% <0.00%> (-1.00%)
...om/alibaba/dubbo/rpc/filter/ActiveLimitFilter.java	77.77% <0.00%> (-11.12%)	5.00% <0.00%> (-2.00%)
...ba/dubbo/remoting/transport/netty/NettyHelper.java	31.25% <0.00%> (-6.25%)	2.00% <0.00%> (ø%)
...onfig/spring/extension/SpringExtensionFactory.java	78.37% <0.00%> (-5.41%)	9.00% <0.00%> (ø%)
... and 9 more

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