We shouldn't use StoragePrice from Blockchain because its dao doesn't
contain the whole set of changes from previouse transactions in the
current block. Instead, we should use an updated storage price for
each transaction and retrieve the price from cached DAO.
The usage of the Blockchain's one leads to the same ExecFeeFactor within
a single block. What we need is to update ExecFeeFactor after each
transaction invocation, thus, cached DAO should be used as it contains
all relevant changes.
We have additional logic for getting BaseExecFee policy value. This
logic should be moved to interop context instead of being in Policer,
because Policer is just an interface over Policy contract.
After moving this logic to interop context, we need to use it to define
BaseExecFee instead of (Policer).BaseExecFee. Thus, moving
(*Blockchain).GetPrice to (*Context).GetPrice is necessary.
We were accepting transactions with zero system fee, but we shouldn't do
that. Also, transaction's verification execution has to be limited by network
fee.
