IMPALA-1186: Use verbose explain level in the query profile.

The fix for IMPALA-921 had broken our existing behavior because it set
a default value for the explain_level query option in the .thrift file.
This patch now forces a verbose explain level in the query profile for
all non-explain statements.

Change-Id: I7b0f6a73ab85bdcd8b70f08115f5b2c4238e5993
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4236
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins