Make the stream async-iterable

[taleinat]

Since node v10, ReadableStreams are async-iterable, i.e. one can do the following:

for await (const chunk of readable) {
    data += chunk;
}

See the docs for more details.

It would be very convenient to be able to iterate through changes this way:

const ChangesStream = require('changes-stream');

const changes = new ChangesStream({ db: '...' });
for await (const change of changes) {
  ...
}