Impala should acquire an ACID lock when dropping a database

Dropping a table takes an exclusive lock on the table, hence it cannot be removed while other queries are working on it.

However, dropping a database doesn't take any locks, so it just starts removing table data even if there are locks on the table by other clients.

Moreover, the Hive Metastore doesn't delete the locks that refer to dropped entities and when the database and tables are re-created those locks will be still around (because the locks refer to the databases and tables via their names).

Impala should acquire an exclusive lock on the database during DROP.