Investigate computing and populating the rewards at the epoch boundary

[kderme]

The ledger incrementaly computes the rewards and make them available through the Reward event. DBSync extracts these events and stores them (one small batch per block) in the db. It may make sence to force the whole computation similarly to what we do for the drep incremental calculation.

Pros:

• rewards will be available earlier, ie the moment that the incremental computation is prepared instead of when it's finished.
• syncing speed may be improved. This is because the incremental computation may have many small queries. When we force the whole computation, we can have less queries with more entries each time, which is in general faster. To validate this, we should investigate the current size of queries.

Cons:

• It can cause delays in new epoch boundaries when following the tip, since db-sync will have to suddenly insert a big batch of data. Also something to investigate.