[SPARK-22824] Restore old offset for binary compatibility

[jose-torres]

What changes were proposed in this pull request?

Some users depend on source compatibility with the org.apache.spark.sql.execution.streaming.Offset class. Although this is not a stable interface, we can keep it in place for now to simplify upgrades to 2.3.

[SparkQA]

Test build #85074 has finished for PR 20012 at commit 7d1df98.

• This patch fails RAT tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #85075 has finished for PR 20012 at commit c94699e.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #85076 has finished for PR 20012 at commit ace608d.

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

[SparkQA]

Test build #85088 has finished for PR 20012 at commit 97fd2f9.

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

[zsxwing]

LGTM. Thanks! Merging to master!