Reserve contract for custom tokens

[kushti]

Currently, reserves may be in ERG only. Would be good to have reserves in custom tokens (SigUSD etc as well).

So the goal of the issue it to have reserve contract (reserve.es) alternative with custom token based reserve instead of ERG. As still the only oracle is Gold/ERG , there is need to get token/ERG price in addition, likely from Spectrum pools as e.g. DuckPools is doing.

Do redemption test in ChainCashSpec as well