Fix row_cache_test
Fix row_cache_test

partition_range passed to row_cache::make_reader
has to be kept alive as long as the resulting reader
is used.

Otherwise weird things start to happen.

This used to work just because of a pure luck.
When I started changing the row_cache implementation
I run into very weird behaviors for this tests.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <2c9e337dbbcf35f4e1394cad043eda10b8c2bd4a.1478602876.git.2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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