## What changes were proposed in this pull request?

In Spark 1.6 and earlier, we can drop the database we are using. In Spark 2.0, native implementation prevent us from dropping current database, which may break some old queries. This PR would re-enable the feature.
## How was this patch tested?

one new unit test in `SessionCatalogSuite`.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #15011 from adrian-wang/dropcurrent.