bytes_ostream: use larger allocations
bytes_ostream: use larger allocations

A 1MB response will require 2000 allocations with the current 512-byte
chunk size.  Increase it exponentially to reduce allocation count for
larger responses (still respecting the upper limit).
Message-Id: <fb2b255b705b591e4da2687af507bce23e1a8fb7@scylladb.com>

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