Store more constants in RlayToken

[hobofan]

Some constants that are used in rlay-client for the payout calculation are currently inferred there. They should instead be stored inside the RlayToken contract and retrieved by rlay-client from there. This should make it easier to build alternative clients and also possibly impose smart-contract side checks that ensure that they are being used correctly.

Constants:

• EPOCH_LENGTH
• EPOCH_START_BLOCK (should be the block the smart contract was deployed in)

[hobofan]

EPOCH_START_BLOCK is saved since 79da17a