Drop database locked the server
Your conclusion is correct about the debian-sys-maint process.  That's not technically part of MySQL and can be disabled.

Your DROP DATABASE gave the appearance of a lockup because opening/closing/dropping tables uses a global mutex in MySQL -- only one such operation can be happening at any point in time and every other similar operation has to wait.  That wouldn't really be a problem, except that MySQL has to wait for the filesystem to report that the underlying table file(s) has been deleted, which can take a significant amount of time.

Dropping the tables individually won't eliminate the problem, but will spread out multiple, shorter blocking events over more time and be potentially less disruptive... or you might be able to use a different filesystem if dropping big tables is a frequent requirement for your environment:

http://www.mysqlperformanceblog.com/2009/06/16/slow-drop-table/ 
https://stackoverflow.com/questions/150058/mysql-drop-database-takes-time-why