Implement lazy retrieval of series from object store.

The bucket store fetches series in a single blocking operation from
object storage. This is likely not an ideal strategy when it comes to
latency and resource usage. In addition, it causes the store to buffer
everything in memory before starting to send results to queriers.

This commit modifies the series retrieval to use the proxy response heap
and take advantage of the k-way merge used in the proxy store.

Signed-off-by: Filip Petkovski <[email protected]>