gms: Don't keep references to reallocated vector entries
gms: Don't keep references to reallocated vector entries

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 <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200422182304.120906-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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