s3: rpc_server: Store new association groups in the id tree

s3: rpc_server: Store new association groups in the id tree

Right now a new association group is created for each connection
assigning the legacy 0x53F0 id, but it is not stored anywhere. When a
second client request to join an association group by its id it is not
found and a new one is created with the same ID.

In practise, it means the association groups are not working even in the
same server process.

This commit stores the created association group in the idtree, but to
make use of it assigns a random id instead of the historical 0x53F0.

The test assoc_group_ok2 was wrongly passing before this change because
the same id 0x53F0 was assigned to all association groups.

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>