flat_mutation_reader: add timeout parameter to operator()()
flat_mutation_reader: add timeout parameter to operator()()

For consistency with fast_foward_to() and fill_buffer(), and for
correctness: operator()() calls fill_buffer() and thus should provide a
timeout for the storage layer.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <6e97552ac2372e5846c955d94400b5315dbd2a89.1536657041.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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