bytes_ostream: Avoid recursion when freeing chunks
bytes_ostream: Avoid recursion when freeing chunks

When there is a lot of chunks we may get stack overflow.

This seems to fix issue #906, a memory corruption during schema
merge. I suspect that what causes corruption there is overflowing of
the stack allocated for the seastar thread. Those stacks don't have
red zones which would catch overflow.

Message-Id: <bb5a34d762f51d37c26a8ab87e14848d3d5adaf4@scylladb.com>

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