sstables: make random_access_reader work with read ahead
sstables: make random_access_reader work with read ahead

Scylla crashes if read ahead is enabled by file_random_access_reader
because a call to seek() destroys the existing input stream without
closing it first.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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