[SPARK-22843][R] Adds localCheckpoint in R

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to add localCheckpoint(..) in R API.

df <- localCheckpoint(createDataFrame(iris))

How was this patch tested?

Unit tests added in R/pkg/tests/fulltests/test_sparkSQL.R

[HyukjinKwon]

cc @felixcheung, could you take a look when you have some time?

[SparkQA]

Test build #85364 has finished for PR 20073 at commit ec27a80.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[felixcheung]

Yes LGTM Btw is there a better way to test localCheckpoint that it is not writing to checkpointDir?

[HyukjinKwon]

I tried to check if there is a easier way but not sure. Scala side seems checking the plan as well but I think I can't simply do it in R side and don't think it's worth ..

[felixcheung]

Fair enough. Calling it, is good enough.

[HyukjinKwon]

Merged to master.

Thank you @felixcheung.