commitlog: drop segment gate
commitlog: drop segment gate

The code that enters the gate never defers before leaving, so the gate
behaves like a flag. Lets use existing flag to prohibit adding data to a
closed segment.
Message-Id: <fa75cfa05c35c538f48c3de97cdeb413cd5e4347@scylladb.com>

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