DROP DATABASE statement writes changes to mysql.proc table under RBR

When replicating a DROP DATABASE statement with a database holding
stored procedures, the changes to the mysql.proc table was recorded
in the binary log under row-based replication.

With this patch, the thread uses statement-logging format for the
duration of the DROP DATABASE statement. The logging format is
(already) reset at the end of the statement, so no additional code
for resetting the logging format is necessary.