After the previous patch replaced all SCYLLA_ASSERT() calls by
throwing_assert(), this patch also replaces all calls to abort().
All these abort() calls are supposedly cases that can never happen,
but if they ever do happen because of a bug, in none of these places
we absolutely need to crash - and exception that aborts the current
operation should be enough.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>