managed_bytes: improve alignment hygene
managed_bytes: improve alignment hygene

While blob_storage is marked as an unaligned type, the back references also
point to an unaligned type (a pointer to blob_storage), since a back
reference can live in a blob_storage.  This triggers errors from zapcc/clang 4.

Fix by creating a type for the reference, which is marked as unaligned.
Message-Id: <643162123c1659fb1922a87170ab9bbb7dc1323a@scylladb.com>

scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla