shard_reader_v2: do_fill_buffer: maybe yield when copying result
shard_reader_v2: do_fill_buffer: maybe yield when copying result

Prevent a reactor stall with e.g. large number of range tombstones.

Fixes #10314

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20220402130847.625085-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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