make loadbalance robust, add comments

[luchy0120]

What is the purpose of the change

This patch can make random choose in LoadBalance more robust .

As the getWeight method in AbstractLoadBalance uses System.currentTimeMillis(),
so in the random choose stage of RandomLoadBalance ,the weight may change
as System.currentTimeMillis() will get new time stamp.

simplified a little bit code in LeastActiveLoadBalance

make getweight always return weight above zero

Also add comments for AbstractLoadBalance

Brief changelog

org.apache.dubbo.rpc.cluster.loadbalance.AbstractLoadBalance
org.apache.dubbo.rpc.cluster.loadbalance.RandomLoadBalance
org.apache.dubbo.rpc.cluster.loadbalance.LeastActiveLoadBalance
org.apache.dubbo.rpc.cluster.loadbalance.RoundRobinLoadBalance

Verifying this change

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2fad342). Click here to learn what that means.
The diff coverage is 92.85%.

@@            Coverage Diff            @@
##             master    #2962   +/-   ##
=========================================
  Coverage          ?   64.05%           
=========================================
  Files             ?      584           
  Lines             ?    26062           
  Branches          ?     4562           
=========================================
  Hits              ?    16694           
  Misses            ?     7186           
  Partials          ?     2182

Impacted Files	Coverage Δ
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	78.12% <ø> (ø)
...rpc/cluster/loadbalance/RoundRobinLoadBalance.java	87.69% <ø> (ø)
...o/rpc/cluster/loadbalance/AbstractLoadBalance.java	47.05% <0%> (ø)
...pc/cluster/loadbalance/LeastActiveLoadBalance.java	88.88% <100%> (ø)
...bbo/rpc/cluster/loadbalance/RandomLoadBalance.java	90% <100%> (ø)

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