Implement single-key iterator

[waynexia]

Currently, the only user-facing iterator implementation is TimeIterator. Which holds lots of underlying internal iterators to gather data from shards. It works like this:

(ts 0, key 1) -> (ts 1, key 1) -> (ts 2, key 1) -> ...

This works fine. However, when we only want to read data from a single key various in timestamp, it is better to get rid of other unnecessary internals and make the rest work simply.