Cleaned up ApphookPool class:

- reordered class methods to better reflect class functionality
- method register(self, app) now doesn't check class supplied by user, just to see it's already registered
- cleaned self.block_register True/False assignments in discover_apps()
- removed discovered check from discover_apps since it has nothig to do with app discovery as such
- changed some exception messages
Fixed typo in get_apphooks() method
More changes in ApphookPool class
Fixed: SyntaxError: invalid syntax
Fixed adding redundant apphooks when APPHOOKS is set
Fixed adding redundant apphooks when APPHOOKS is set
Rename argument in register method to be more self-explanatory
Removed direct reference to apphook_pool fields. Now they're accessed through get_apphooks and get_apphook methoeds.
Fixed typo - remove one apphook_pool from calling chain
Merge pull request #3101 from mmarzantowicz/apphook-cleanup

Apphook cleanup