breaks SBR

The problem was that DROP DATABASE ignored any metadata locks on stored
functions and procedures held by other connections. This made it
possible for DROP DATABASE to drop functions/procedures that were in use
by other connections and therefore break statement based replication.
(DROP DATABASE could appear in the binlog before a statement using a
dropped function/procedure.)

This problem was an issue left unresolved by the patch for Bug#30977
where metadata locks for stored functions/procedures were introduced.

This patch fixes the problem by making sure DROP DATABASE takes
exclusive metadata locks on all stored functions/procedures to be
dropped.

Test case added to sp-lock.test.