Bluetooth: CSIS: Stored actives members by rank

Instead of getting doing look ups for each member
for the CSIS client procedures, we store the active
members ordered by rank.

This is slightly less efficient in cases of error,
but for some procedures as the upcoming Ordered
access procedure where we need to access each
member twice, ordering them once is more
efficient.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>