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
This will make it easier to access table proprties in places which
only have schema_ptr. This is in particular useful when replacing
dht::shard_of() uses with s->table().shard_of(), now that sharding is
no longer static, but table-specific.
Also, it allows us to install a guard which catches invalid uses of
schema::get_sharder() on tablet-based tables.
It will be helpful for other uses as well. For example, we can now get
rid of the static_props hack.
System tables have static schemas and code uses those static schemas
instead of looking them up in the database. We want those schemas to
have a valid table() once the table is created, so we need to attach
registry entry to the target schema rather than to a schema duplicate.
Schema related files are moved there. This excludes schema files that
also interact with mutations, because the mutation module depends on
the schema. Those files will have to go into a separate module.
Closes#12858
