row_cache: make_reader: make read_context only when needed
row_cache: make_reader: make read_context only when needed

So we can have better control on who's responsible to close it.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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