Merge #8298

8298: Stop using an upgradeable read lock in interning r=jonas-schievink a=jonas-schievink

Only one upgradeable read lock can be handed out at the same time, and
we never acquire a non-upgradeable read lock, so this has no benefit
over just using a write lock in the first place.

bors r+

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>