sharded_store's main purpose is to supply a consistent interface
to an underlying (sharded) in-memory store. So to process a given
request, it must first work out the home shard for the desired
schema ID or subject, dispatching requests to that shard.

Recently, schema registry was refactored to store schema definitions
as iobufs (rather than contiguous strings) to avoid large allocations.
To avoid semantically unnecessary _copies_, get_schema_definition was
implemented to return a iobuf share across a shard boundary. This is
an unsafe operation, as the reference counting and deletion machinery
of the underlying temporary buffers is totally unsynchronized. This is
UB in principle (data race) and leads to memory access faults (UAF, etc.)
in practice.

So this commit changes 'store::get_schema_definition' to return a copy
of the stored schema definition (iobuf), eliminating any cross-shard
sharing of the underlying temp buffers. This approach is rather blunt
and carries some cost at deallocation time (seastar allocator must
work out the appropriate site for memory reclamation for each component
temp buffer of a given schema def), but it should be safe since the
component temp buffers are never live on multiple shards simultaneously.

Note that this path could be optimized somewhat by returning a shared
iobuf managed by foreign_ptr and carrying out the necessary copy (a
read-only operation) on the requesting shard.

This commit also includes a "unit test" that is more of a memory access
canary, of sorts. Reverting the change in 'store::gets_schema_definition'
will cause the test to fail reliably, producing memory faults in line with
the reasoning above. With the change in place, this test reliably runs to
completion without issue.

Signed-off-by: Oren Leiman <oren.leiman@redpanda.com>
(cherry picked from commit 420bd28)