discover its existence".

The problem was that user without any privileges on 
routine was able to find out whether it existed or not.
DROP FUNCTION and DROP PROCEDURE statements were 
checking if routine being dropped existed and reported 
ER_SP_DOES_NOT_EXIST error/warning before checking 
if user had enough privileges to drop it.

This patch solves this problem by changing code not to 
check if routine exists before checking if user has enough 
privileges to drop it. Moreover we no longer perform this 
check using a separate call instead we rely on 
sp_drop_routine() returning SP_KEY_NOT_FOUND if routine 
doesn't exist.

This change also simplifies one of upcoming patches
refactoring global read lock implementation.