* add test for validate multi reward periods
* tidy up: combine files
* don't accumulate global indexes containing zeros
Previously if the time since last block was 0,
indexes were added containing 0s.
Now leave them out. Missing is assumed to be 0.
* move state independent test to types folder
* clarify reward source concept to "source shares"
- rename variables and update doc comments
- extract method from swap accumulation
* tidy up and expand swap accumulation unit tests
* rename swap test file to match others
* update swap pool id format in tests
* refactor borrow accumulation, use new accumulator
* refactor supply accumulation, use new accumulator
* refactor delegator accumulation, use accumulator
* refactor usdx accumulation, use new accumulator
* fix types const
* remove unsed methods
* more usdx minting param validation.
Protect against the rewards per second denom changing.
It should always be "ukava".
* add safety check in InitGenesis
It prevents huge accumulations on the first block by limiting all
previous accumulation times to be within one year of genesis
* add todo for adding swp token distirbution info
