Adding tests for batch execution requests

[Maelkum]

This PR needs rebasing after merging #201.

Tests currently cover:

• correct handling of incoming batch execution requests (saving batch and work items in the batch store)
• correct creation of new chunks and updating work items (to add chunk info)
• work partitioning to a set of nodes

PR includes a few fixes and tweaks:

• corrected handling of max_attempts setting (saved to DB)
• when we have more nodes than we need, we correctly drop nodes that do not have any work items assigned to them (as they have nothing to execute)