More efficient group share resolving

When resolving a group share to the user group share we used to do this
on a per share basis. Now we try to do this for all group shares at
once. Of course still chunked.

Before: N incomming group shares this would mean 1 + N queries
Now: N incomming roups shares this would mean 1 + 1 queries

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>