feat: add getOperation([seq]) API

[Nuhvi]

This is useful for example for watching operations on the bee

db.feed.on('download', (seq) => db.getOperation(seq))

Or

db.feed.on('append', () => db.feed.length > 1 && db.getOperation(db.feed.length - 1))

Also makes #23 a bit easier to achieve.

[mafintosh]

@Nazeh LGTM on the change requested and fixing the conflict