These callbacks can block a seastar thread and the underlying vector
can be reallocated concurrently.
This is no different than if it was a plain std::vector and the
solution is similar: use values instead of references.
Fixes#6230
Signed-off-by: Rafael Ávila de Espíndola <espindola@scylladb.com>
Message-Id: <20200422182304.120906-1-espindola@scylladb.com>