Add tier information to payment logs when allocating tokens.

[csmuller]

For better transparency of the token allocation, we could also add tier information to the payment logs. I.e. for every payment log store the information of which tier the allocated tokens have been taken. Without this it is not clear what discount has been applied when allocating the tokens.

Since for one payment tokens can be drawn from multiple tiers (in case it overflow the one tier) a new table would be needed. The tables entries reference a payment log and store token amounts and the tier from which the tokens where drawn from.