Not just bytes::output_iterator. Allow writing into streams other than
just `bytes`. In fact we should be very careful with writing into
`bytes` as they require potentially large contiguous allocations.
The `write()` method is now templatized also on the type of its first
argument, which now accepts any CharOutputIterator. Due to our poor
usage of namespace this now collides with `write` defined inside
`db/commitlog/commitlog.cc`. Luckily, the latter doesn't really have to
be templatized on the data type it reads from, and de-templatizing it
resolves the clash.
4054ba0c45