store GetEmptyTicksForEpochs should use an Iter

[0xluk]

What? Why? Who?

Due to the fact that we had performance issues in the /v1/status endpoint identified in #83, after we narrowed it down, we came to the conclusion that the GetEmptyTicksForEpochs was taking a lot of CPU time when an archiver instance has a lot of epochs stored.

How?

Instead of using multiple Get in a for loop, getting multiple epochs (keys) empty ticks should be done by using an iterator which is far more efficient.

Acceptance Criteria

The same data as before needs to be returned and hotfix implementation(/v1/status returns only current tick of the epoch now, included here: 36d5326) needs to be reverted