BACKGROUND:
In certain situations DROP USER fails to remove all privileges
belonging to user being dropped from in-memory structures.
Current workaround is to do DROP USER twice in scenario below
OR doing FLUSH PRIVILEGES after doing DROP USER.

ANALYSIS:
In MySQL, When we grant some stored routines privileges to a
user they are stored in their respective hash.
When doing DROP USER all the stored routine privilege entries
associated with that user has to be deleted from its respective 
hash.
The root cause for this bug is some entries from the hash
are not getting deleted. 
The problem is that code that deletes entries from the hash tries
to do so while iterating over it, without taking enough measures
to address the fact that such deletion can reshuffle elements in 
the hash. If the user/administrator creates the same user again 
he is thrown an  error 'Error 1396 ER_CANNOT_USER' from MySQL.
This prompts the user to either do FLUSH PRIVILEGES or do DROP USER 
again. This behaviour is not desirable as it is a workaround and
does not solves the problem mentioned above.

FIX:
This bug is fixed by introducing a dynamic array to store the 
pointersto all stored routine privilege objects that either have
to be deleted or updated. This is done in 3 steps.
Step 1: Fetching the element from the hash and checking whether 
it is to be deleted or updated.
Step 2: Storing the pointer to that privilege object in dynamic array.
Step 3: Traversing the dynamic array to perform the appropriate action 
either delete or update.
This is a much cleaner way to delete or update the privilege entries 
associated with some user and solves the problem mentioned above.
Also the code has been refactored a bit by introducing an enum
instead of hard coded numbers used for respective dynamic arrays 
and hashes in handle_grant_struct() function.