Before this commit, there where two different locks when updating the restricted
data cache. A future lock, what is faster but only works in the same thread. The
other lock is in redis, it is not so fast, but also works in many threads.
The future lock was buggy, because on a second call of update_restricted_data
the same future was reused. So on the second run, the future was already done.
I don't see any way to delete. The last client would have to delete it, but there
is no way to find out which client the last one is.
