The underlying data source for cache should not be the same memtable
which is later used to update the cache from. This fixes the following
assertion failure:
row_cache_test_g: utils/logalloc.hh:289: decltype(auto) logalloc::allocating_section::operator()(logalloc::region&, Func&&) [with Func = memtable::make_reader(schema_ptr, const partition_range&)::<lambda()>]: Assertion `r.reclaiming_enabled()' failed.
The problem is that when memtable is merged into cache their regions
are also merged, so locking cache's region locks the memtable region
as well.
Since bytes is a very generic value that is returned from many calls,
it is easy to pass it by mistake to a function expecting a data_value,
and to get a wrong result. It is impossible for the data_value constructor
to know if the argument is a genuine bytes variable, a data_value of another
type, but serialized, or some other serialized data type.
To prevent misuse, make the data_value(bytes) constructor
(and complementary data_value(optional<bytes>) explicit.
