Implement TRUNCATE for insert-only ACID tables

Impala currently cannot TRUNCATE insert-only tables.

TRUNCATE is a DDL statement that deletes all the files and drops all column and table statistics. (Impala currently cannot truncate specific partitions, only the whole table. Truncating specific partitions is out of scope of this Jira.)

TRUNCATE doesn't only mean to create a new empty base directory, but to really remove all the files, this is the behavior of Hive as well.

To implement TRUNCATE Impala must acquire an EXCLUSIVE lock on the table. After that Impala must recursively delete all the data files belonging to the table.