Speed up lyskom-get-membership (a lot)

Detailed changes:
> 	* reading.el (lyskom-add-memberships-to-membership): Use
> 	lyskom-membership-table.
> 	(lyskom-insert-memberships-in-membership): Ditto.
> 	(lyskom-do-insert-membership): Ditto.
> 	(lyskom-do-remove-membership): Ditto.
>
> 	Speed up lyskom-get-membership (a lot):
> 	* lyskom-rest.el (lyskom-membership-table): New variable
> 	(lyskom-membership-table-add): New function.
> 	(lyskom-membership-table-del): New function.
> 	(lyskom-try-get-membership): Use lyskom-membership-table.
>