Fix block production timing logic

[FlorianFranzen]

Current Behavior

The timing logic used during block production is not optimal at the moment:

• You probably should not use the full slot time to collect extrinsics. For example substrate reserves the last 1/3 of the slot time for finalization of the block.

• You should not abort collecting extrinsics if the queue is or becomes empty, but continue to include new transaction throughout the slot to maximize block utilization.

(Also, why would an extrinsic be empty? Should this at least cause a warning?)

Expected Behavior

• Extrinsics should be continued to be included into the block till about 2/3 of the slot time.