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ARROW-5842: [Java] Revise the semantic of lastSet in ListVector

The lastSet member in ListVector seems misleading. According to the name, it should refers to the last index that is actually set. However, from the context of the code, it actually means the next index that will be set.

We fix this problem, and make it consistent with the lastSet in BaseVariableWidthVector.

Author: liyafan82 <fan_li_ya@foxmail.com>

Closes #4797 from liyafan82/fly_0704_lastset and squashes the following commits:

6043263d9 <liyafan82>  Revise external semantics
56a9ef336 <liyafan82>  Revise the semantic of lastSet in ListVector