mutation_partition_visitor: Fix crash for large blobs
mutation_partition_visitor: Fix crash for large blobs

Fixes #927.

The new visiting code builds cell instances using
atomic_cell::make_*() factory methods, which won't work in LSA context
because they depend on managed_bytes storage to be linearized. It may
not be since large blob support. This worked before because we created
cells from views before which works in all contexts.

Fix by constructing them in standard allocator context.

Message-Id: <30d7a03012623c408b955acd016342b7bee3c41e@scylladb.com>

duarten/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla