The schema registry disarms internal timers when it is destroyed.
This accesses the Seastar reactor. However, after [1] we don't have ordering
between the reactor destruction and the thread_local registry destruction.
Fix this by flushing all entries when the database is destroyed. The
database object is fundamental so it's unlikely we'll have anything
using the registry after it's gone.
[1] 101b245ed7
c00b376a3e