[SPARK-15208][WIP][CORE][STREAMING][DOCS] Update Spark examples with AccumulatorV2

## What changes were proposed in this pull request?

The patch updates the codes & docs in the example module as well as the related doc module:

- [ ] [docs] `streaming-programming-guide.md`
  - [x] scala code part
  - [ ] java code part
  - [ ] python code part
- [x] [examples] `RecoverableNetworkWordCount.scala`
- [ ] [examples] `JavaRecoverableNetworkWordCount.java`
- [ ] [examples] `recoverable_network_wordcount.py`

## How was this patch tested?

Ran the examples and verified results manually.

Author: Liwei Lin <lwlin7@gmail.com>

Closes #12981 from lw-lin/accumulatorV2-examples.