[SPARK-20920][SQL] ForkJoinPool pools are leaked when writing hive tables with many partitions

## What changes were proposed in this pull request?

Don't leave thread pool running from AlterTableRecoverPartitionsCommand DDL command

## How was this patch tested?

Existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #18216 from srowen/SPARK-20920.